lsqnonlin

Solve nonlinear least-squares (nonlinear data-fitting) problems

Syntax

x = lsqnonlin(fun,x0)

x = lsqnonlin(fun,x0,lb,ub)

x = lsqnonlin(fun,x0,lb,ub,options)

x = lsqnonlin(problem)

[x,resnorm]
= lsqnonlin(___)

[x,resnorm,residual,exitflag,output]
= lsqnonlin(___)

[x,resnorm,residual,exitflag,output,lambda,jacobian]
= lsqnonlin(___)

Description

Nonlinear least-squares solver

Solves nonlinear least-squares curve fitting problems of the form

$\min_{x} {‖ f (x) ‖}_{2}^{2} = \min_{x} (f_{1} {(x)}^{2} + f_{2} {(x)}^{2} + ... + f_{n} {(x)}^{2})$

with optional lower and upper bounds lb and ub on the components of x.

x, lb, and ub can be vectors or matrices; see Matrix Arguments.

Rather than compute the value ${‖ f (x) ‖}_{2}^{2}$ (the sum of squares), lsqnonlin requires the user-defined function to compute the vector-valued function

$f (x) = [\begin{matrix} f_{1} (x) \\ f_{2} (x) \\ ⋮ \\ f_{n} (x) \end{matrix}] .$

example

x = lsqnonlin(fun,x0) starts at the point x0 and finds a minimum of the sum of squares of the functions described in fun. The function fun should return a vector (or array) of values and not the sum of squares of the values. (The algorithm implicitly computes the sum of squares of the components of fun(x).)

Note

Passing Extra Parameters explains how to pass extra parameters to the vector function fun(x), if necessary.

example

x = lsqnonlin(fun,x0,lb,ub) defines a set of lower and upper bounds on the design variables in x, so that the solution is always in the range lb ≤ x ≤ ub. You can fix the solution component x(i) by specifying lb(i) = ub(i).

Note

If the specified input bounds for a problem are inconsistent, the output x is x0 and the outputs resnorm and residual are [].

Components of x0 that violate the bounds lb ≤ x ≤ ub are reset to the interior of the box defined by the bounds. Components that respect the bounds are not changed.

example

x = lsqnonlin(fun,x0,lb,ub,options) minimizes with the optimization options specified in options. Use optimoptions to set these options. Pass empty matrices for lb and ub if no bounds exist.

x = lsqnonlin(problem) finds the minimum for problem, a structure described in problem.

example

[x,resnorm] = lsqnonlin(___), for any input arguments, returns the value of the squared 2-norm of the residual at x: sum(fun(x).^2).

example

[x,resnorm,residual,exitflag,output] = lsqnonlin(___) additionally returns the value of the residual fun(x) at the solution x, a value exitflag that describes the exit condition, and a structure output that contains information about the optimization process.

[x,resnorm,residual,exitflag,output,lambda,jacobian] = lsqnonlin(___) additionally returns a structure lambda whose fields contain the Lagrange multipliers at the solution x, and the Jacobian of fun at the solution x.

Examples

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Fit a Simple Exponential

Open Live Script

Fit a simple exponential decay curve to data.

Generate data from an exponential decay model plus noise. The model is

$y = \exp (- 1.3 t) + ε,$

with $t$ ranging from 0 through 3, and $ε$ normally distributed noise with mean 0 and standard deviation 0.05.

rng default % for reproducibility
d = linspace(0,3);
y = exp(-1.3*d) + 0.05*randn(size(d));

The problem is: given the data (d, y), find the exponential decay rate that best fits the data.

Create an anonymous function that takes a value of the exponential decay rate $r$ and returns a vector of differences from the model with that decay rate and the data.

fun = @(r)exp(-d*r)-y;

Find the value of the optimal decay rate. Arbitrarily choose an initial guess x0 = 4.

x0 = 4;
x = lsqnonlin(fun,x0)

Local minimum possible.

lsqnonlin stopped because the final change in the sum of squares relative to 
its initial value is less than the value of the function tolerance.

x = 1.2645

Plot the data and the best-fitting exponential curve.

plot(d,y,'ko',d,exp(-x*d),'b-')
legend('Data','Best fit')
xlabel('t')
ylabel('exp(-tx)')

Fit a Problem with Bound Constraints

Open Live Script

Find the best-fitting model when some of the fitting parameters have bounds.

Find a centering $b$ and scaling $a$ that best fit the function

$a \exp (- t) \exp (- \exp (- (t - b)))$

to the standard normal density,

$\frac{1}{\sqrt{2 π}} \exp (- t^{2} / 2) .$

Create a vector t of data points, and the corresponding normal density at those points.

t = linspace(-4,4);
y = 1/sqrt(2*pi)*exp(-t.^2/2);

Create a function that evaluates the difference between the centered and scaled function from the normal y, with x(1) as the scaling $a$ and x(2) as the centering $b$ .

fun = @(x)x(1)*exp(-t).*exp(-exp(-(t-x(2)))) - y;

Find the optimal fit starting from x0 = [1/2,0], with the scaling $a$ between 1/2 and 3/2, and the centering $b$ between -1 and 3.

lb = [1/2,-1];
ub = [3/2,3];
x0 = [1/2,0];
x = lsqnonlin(fun,x0,lb,ub)

Local minimum possible.

lsqnonlin stopped because the final change in the sum of squares relative to 
its initial value is less than the value of the function tolerance.

x = 1×2

    0.8231   -0.2444

Plot the two functions to see the quality of the fit.

plot(t,y,'r-',t,fun(x)+y,'b-')
xlabel('t')
legend('Normal density','Fitted function')

Nonlinear Least Squares with Nondefault Options

Open Live Script

Compare the results of a data-fitting problem when using different lsqnonlin algorithms.

Suppose that you have observation time data xdata and observed response data ydata, and you want to find parameters $x (1)$ and $x (2)$ to fit a model of the form

$ydata = x (1) \exp (x (2) xdata) .$

Input the observation times and responses.

xdata = ...
 [0.9 1.5 13.8 19.8 24.1 28.2 35.2 60.3 74.6 81.3];
ydata = ...
 [455.2 428.6 124.1 67.3 43.2 28.1 13.1 -0.4 -1.3 -1.5];

Create a simple exponential decay model. The model computes a vector of differences between predicted values and observed values.

fun = @(x)x(1)*exp(x(2)*xdata)-ydata;

Fit the model using the starting point x0 = [100,-1]. First, use the default 'trust-region-reflective' algorithm.

x0 = [100,-1];
options = optimoptions(@lsqnonlin,'Algorithm','trust-region-reflective');
x = lsqnonlin(fun,x0,[],[],options)

Local minimum possible.

lsqnonlin stopped because the final change in the sum of squares relative to 
its initial value is less than the value of the function tolerance.

x = 1×2

  498.8309   -0.1013

See if there is any difference using the 'levenberg-marquardt algorithm.

options.Algorithm = 'levenberg-marquardt';
x = lsqnonlin(fun,x0,[],[],options)

Local minimum possible.
lsqnonlin stopped because the relative size of the current step is less than
the value of the step size tolerance.

x = 1×2

  498.8309   -0.1013

The two algorithms found the same solution. Plot the solution and the data.

plot(xdata,ydata,'ko')
hold on
tlist = linspace(xdata(1),xdata(end));
plot(tlist,x(1)*exp(x(2)*tlist),'b-')
xlabel xdata
ylabel ydata
title('Exponential Fit to Data')
legend('Data','Exponential Fit')
hold off

Nonlinear Least Squares Solution and Residual Norm

Open Live Script

Find the $x$ that minimizes

$\sum_{k = 1}^{10} {(2 + 2 k - e^{k x_{1}} - e^{k x_{2}})}^{2}$ ,

and find the value of the minimal sum of squares.

Because lsqnonlin assumes that the sum of squares is not explicitly formed in the user-defined function, the function passed to lsqnonlin should instead compute the vector-valued function

$F_{k} (x) = 2 + 2 k - e^{k x_{1}} - e^{k x_{2}}$ ,

for $k = 1$ to $10$ (that is, $F$ should have $10$ components).

The myfun function that computes the 10-component vector F appears at the end of this example.

Find the minimizing point and the minimum value, starting at the point x0 = [0.3,0.4].

x0 = [0.3,0.4];
[x,resnorm] = lsqnonlin(@myfun,x0)

Local minimum possible.
lsqnonlin stopped because the size of the current step is less than
the value of the step size tolerance.

x = 1×2

    0.2578    0.2578

resnorm = 124.3622

The resnorm output is the squared residual norm, the sum of squares of the function values.

The following function computes the vector-valued objective function.

function F = myfun(x)
k = 1:10;
F = 2 + 2*k-exp(k*x(1))-exp(k*x(2));
end

Examine the Solution Process

Open Live Script

Examine the solution process both as it occurs (by setting the Display option to 'iter') and afterward (by examining the output structure).

Suppose that you have observation time data xdata and observed response data ydata, and you want to find parameters $x (1)$ and $x (2)$ to fit a model of the form

$ydata = x (1) \exp (x (2) xdata) .$

Input the observation times and responses.

xdata = ...
 [0.9 1.5 13.8 19.8 24.1 28.2 35.2 60.3 74.6 81.3];
ydata = ...
 [455.2 428.6 124.1 67.3 43.2 28.1 13.1 -0.4 -1.3 -1.5];

Create a simple exponential decay model. The model computes a vector of differences between predicted values and observed values.

fun = @(x)x(1)*exp(x(2)*xdata)-ydata;

Fit the model using the starting point x0 = [100,-1]. Examine the solution process by setting the Display option to 'iter'. Obtain an output structure to obtain more information about the solution process.

x0 = [100,-1];
options = optimoptions('lsqnonlin','Display','iter');
[x,resnorm,residual,exitflag,output] = lsqnonlin(fun,x0,[],[],options);

                                         Norm of      First-order 
 Iteration  Func-count     f(x)          step          optimality
     0          3          359677                      2.88e+04
Objective function returned Inf; trying a new point...
     1          6          359677        11.6976       2.88e+04      
     2          9          321395            0.5       4.97e+04      
     3         12          321395              1       4.97e+04      
     4         15          292253           0.25       7.06e+04      
     5         18          292253            0.5       7.06e+04      
     6         21          270350          0.125       1.15e+05      
     7         24          270350           0.25       1.15e+05      
     8         27          252777         0.0625       1.63e+05      
     9         30          252777          0.125       1.63e+05      
    10         33          243877        0.03125       7.48e+04      
    11         36          243660         0.0625        8.7e+04      
    12         39          243276         0.0625          2e+04      
    13         42          243174         0.0625       1.14e+04      
    14         45          242999          0.125        5.1e+03      
    15         48          242661           0.25       2.04e+03      
    16         51          241987            0.5       1.91e+03      
    17         54          240643              1       1.04e+03      
    18         57          237971              2       3.36e+03      
    19         60          232686              4       6.04e+03      
    20         63          222354              8        1.2e+04      
    21         66          202592             16       2.25e+04      
    22         69          166443             32       4.05e+04      
    23         72          106320             64       6.68e+04      
    24         75         28704.7            128       8.31e+04      
    25         78         89.7947        140.674       2.22e+04      
    26         81         9.57381        2.02599            684      
    27         84         9.50489      0.0619927           2.27      
    28         87         9.50489    0.000462262         0.0114      

Local minimum possible.

lsqnonlin stopped because the final change in the sum of squares relative to 
its initial value is less than the value of the function tolerance.

Examine the output structure to obtain more information about the solution process.

output

output = struct with fields:
    firstorderopt: 0.0114
       iterations: 28
        funcCount: 87
     cgiterations: 0
        algorithm: 'trust-region-reflective'
         stepsize: 4.6226e-04
          message: '...'

For comparison, set the Algorithm option to 'levenberg-marquardt'.

options.Algorithm = 'levenberg-marquardt';
[x,resnorm,residual,exitflag,output] = lsqnonlin(fun,x0,[],[],options);

                                        First-Order                    Norm of 
 Iteration  Func-count    Residual       optimality      Lambda           step
     0           3          359677        2.88e+04         0.01
Objective function returned Inf; trying a new point...
     1          13          340761        3.91e+04       100000       0.280777
     2          16          304661        5.97e+04        10000       0.373146
     3          21          297292        6.55e+04        1e+06      0.0589933
     4          24          288240        7.57e+04       100000      0.0645444
     5          28          275407        1.01e+05        1e+06      0.0741266
     6          31          249954        1.62e+05       100000       0.094571
     7          36          245896        1.35e+05        1e+07      0.0133606
     8          39          243846        7.26e+04        1e+06     0.00944311
     9          42          243568        5.66e+04       100000     0.00821621
    10          45          243424        1.61e+04        10000     0.00777935
    11          48          243322         8.8e+03         1000      0.0673933
    12          51          242408         5.1e+03          100       0.675209
    13          54          233628        1.05e+04           10        6.59804
    14          57          169089        8.51e+04            1        54.6992
    15          60         30814.7        1.54e+05          0.1        196.939
    16          63         147.496           8e+03         0.01        129.795
    17          66         9.51503             117        0.001        9.96069
    18          69         9.50489          0.0714       0.0001       0.080486
    19          72         9.50489        4.91e-05        1e-05    5.07033e-05

Local minimum possible.
lsqnonlin stopped because the relative size of the current step is less than
the value of the step size tolerance.

The 'levenberg-marquardt' converged with fewer iterations, but almost as many function evaluations:

output

output = struct with fields:
       iterations: 19
        funcCount: 72
         stepsize: 5.0703e-05
     cgiterations: []
    firstorderopt: 4.9122e-05
        algorithm: 'levenberg-marquardt'
          message: '...'

Input Arguments

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`fun` — Function whose sum of squares is minimized
function handle | name of function

Function whose sum of squares is minimized, specified as a function handle or the name of a function. fun is a function that accepts an array x and returns an array F, the objective functions evaluated at x. The function fun can be specified as a function handle to a file:

x = lsqnonlin(@myfun,x0)

where myfun is a MATLAB^® function such as

function F = myfun(x)
F = ...            % Compute function values at x

fun can also be a function handle for an anonymous function.

x = lsqnonlin(@(x)sin(x.*x),x0);

If the user-defined values for x and F are arrays, they are converted to vectors using linear indexing (see Array Indexing).

Note

The sum of squares should not be formed explicitly. Instead, your function should return a vector of function values. See Examples.

If the Jacobian can also be computed and the 'SpecifyObjectiveGradient' option is true, set by

options = optimoptions('lsqnonlin','SpecifyObjectiveGradient',true)

then the function fun must return a second output argument with the Jacobian value J (a matrix) at x. By checking the value of nargout, the function can avoid computing J when fun is called with only one output argument (in the case where the optimization algorithm only needs the value of F but not J).

function [F,J] = myfun(x)
F = ...          % Objective function values at x
if nargout > 1   % Two output arguments
   J = ...   % Jacobian of the function evaluated at x
end

If fun returns an array of m components and x has n elements, where n is the number of elements of x0, the Jacobian J is an m-by-n matrix where J(i,j) is the partial derivative of F(i) with respect to x(j). (The Jacobian J is the transpose of the gradient of F.)

Example: @(x)cos(x).*exp(-x)

Data Types: char | function_handle | string

`x0` — Initial point
real vector | real array

Initial point, specified as a real vector or real array. Solvers use the number of elements in x0 and the size of x0 to determine the number and size of variables that fun accepts.

Example: x0 = [1,2,3,4]

Data Types: double

`lb` — Lower bounds
real vector | real array

Lower bounds, specified as a real vector or real array. If the number of elements in x0 is equal to the number of elements in lb, then lb specifies that

x(i) >= lb(i) for all i.

If numel(lb) < numel(x0), then lb specifies that

x(i) >= lb(i) for 1 <= i <= numel(lb).

If there are fewer elements in lb than in x0, solvers issue a warning.

Example: To specify that all x components are positive, use lb = zeros(size(x0)).

Data Types: double

`ub` — Upper bounds
real vector | real array

Upper bounds, specified as a real vector or real array. If the number of elements in x0 is equal to the number of elements in ub, then ub specifies that

x(i) <= ub(i) for all i.

If numel(ub) < numel(x0), then ub specifies that

x(i) <= ub(i) for 1 <= i <= numel(ub).

If there are fewer elements in ub than in x0, solvers issue a warning.

Example: To specify that all x components are less than 1, use ub = ones(size(x0)).

Data Types: double

`options` — Optimization options
output of `optimoptions` | structure as `optimset` returns

Optimization options, specified as the output of optimoptions or a structure as optimset returns.

Some options apply to all algorithms, and others are relevant for particular algorithms. See Optimization Options Reference for detailed information.

Some options are absent from the optimoptions display. These options appear in italics in the following table. For details, see View Options.

All Algorithms
`Algorithm`	Choose between `'trust-region-reflective'` (default) and `'levenberg-marquardt'`. The `Algorithm` option specifies a preference for which algorithm to use. It is only a preference, because certain conditions must be met to use each algorithm. For the trust-region-reflective algorithm, the nonlinear system of equations cannot be underdetermined; that is, the number of equations (the number of elements of `F` returned by `fun`) must be at least as many as the length of `x`. For more information on choosing the algorithm, see Choosing the Algorithm.
`CheckGradients`	Compare user-supplied derivatives (gradients of objective or constraints) to finite-differencing derivatives. Choices are `false` (default) or `true`. For `optimset`, the name is `DerivativeCheck` and the values are `'on'` or `'off'`. See Current and Legacy Option Names.
Diagnostics	Display diagnostic information about the function to be minimized or solved. Choices are `'off'` (default) or `'on'`.
DiffMaxChange	Maximum change in variables for finite-difference gradients (a positive scalar). The default is `Inf`.
DiffMinChange	Minimum change in variables for finite-difference gradients (a positive scalar). The default is `0`.
`Display`	Level of display (see Iterative Display): `'off'` or `'none'` displays no output. `'iter'` displays output at each iteration, and gives the default exit message. `'iter-detailed'` displays output at each iteration, and gives the technical exit message. `'final'` (default) displays just the final output, and gives the default exit message. `'final-detailed'` displays just the final output, and gives the technical exit message.
`FiniteDifferenceStepSize`	Scalar or vector step size factor for finite differences. When you set `FiniteDifferenceStepSize` to a vector `v`, the forward finite differences `delta` are `delta = v.sign′(x).max(abs(x),TypicalX);` where `sign′(x) = sign(x)` except `sign′(0) = 1`. Central finite differences are `delta = v.*max(abs(x),TypicalX);` Scalar `FiniteDifferenceStepSize` expands to a vector. The default is `sqrt(eps)` for forward finite differences, and `eps^(1/3)` for central finite differences. For `optimset`, the name is `FinDiffRelStep`. See Current and Legacy Option Names.
`FiniteDifferenceType`	Finite differences, used to estimate gradients, are either `'forward'` (default), or `'central'` (centered). `'central'` takes twice as many function evaluations, but should be more accurate. The algorithm is careful to obey bounds when estimating both types of finite differences. So, for example, it could take a backward, rather than a forward, difference to avoid evaluating at a point outside bounds. For `optimset`, the name is `FinDiffType`. See Current and Legacy Option Names.
`FunctionTolerance`	Termination tolerance on the function value, a positive scalar. The default is `1e-6`. See Tolerances and Stopping Criteria. For `optimset`, the name is `TolFun`. See Current and Legacy Option Names.
FunValCheck	Check whether function values are valid. `'on'` displays an error when the function returns a value that is `complex`, `Inf`, or `NaN`. The default `'off'` displays no error.
`MaxFunctionEvaluations`	Maximum number of function evaluations allowed, a positive integer. The default is `100*numberOfVariables`. See Tolerances and Stopping Criteria and Iterations and Function Counts. For `optimset`, the name is `MaxFunEvals`. See Current and Legacy Option Names.
`MaxIterations`	Maximum number of iterations allowed, a positive integer. The default is `400`. See Tolerances and Stopping Criteria and Iterations and Function Counts. For `optimset`, the name is `MaxIter`. See Current and Legacy Option Names.
`OptimalityTolerance`	Termination tolerance on the first-order optimality (a positive scalar). The default is `1e-6`. See First-Order Optimality Measure. Internally, the `'levenberg-marquardt'` algorithm uses an optimality tolerance (stopping criterion) of `1e-4` times `FunctionTolerance` and does not use `OptimalityTolerance`. For `optimset`, the name is `TolFun`. See Current and Legacy Option Names.
`OutputFcn`	Specify one or more user-defined functions that an optimization function calls at each iteration. Pass a function handle or a cell array of function handles. The default is none (`[]`). See Output Function and Plot Function Syntax.
`PlotFcn`	Plots various measures of progress while the algorithm executes; select from predefined plots or write your own. Pass a name, a function handle, or a cell array of names or function handles. For custom plot functions, pass function handles. The default is none (`[]`): `'optimplotx'` plots the current point. `'optimplotfunccount'` plots the function count. `'optimplotfval'` plots the function value. `'optimplotresnorm'` plots the norm of the residuals. `'optimplotstepsize'` plots the step size. `'optimplotfirstorderopt'` plots the first-order optimality measure. Custom plot functions use the same syntax as output functions. See Output Functions for Optimization Toolbox™ and Output Function and Plot Function Syntax. For `optimset`, the name is `PlotFcns`. See Current and Legacy Option Names.
`SpecifyObjectiveGradient`	If `false` (default), the solver approximates the Jacobian using finite differences. If `true`, the solver uses a user-defined Jacobian (defined in `fun`), or Jacobian information (when using `JacobMult`), for the objective function. For `optimset`, the name is `Jacobian`, and the values are `'on'` or `'off'`. See Current and Legacy Option Names.
`StepTolerance`	Termination tolerance on `x`, a positive scalar. The default is `1e-6`. See Tolerances and Stopping Criteria. For `optimset`, the name is `TolX`. See Current and Legacy Option Names.
`TypicalX`	Typical `x` values. The number of elements in `TypicalX` is equal to the number of elements in `x0`, the starting point. The default value is `ones(numberofvariables,1)`. The solver uses `TypicalX` for scaling finite differences for gradient estimation.
`UseParallel`	When `true`, the solver estimates gradients in parallel. Disable by setting to the default, `false`. See Parallel Computing.
Trust-Region-Reflective Algorithm
`JacobianMultiplyFcn`	Jacobian multiply function, specified as a function handle. For large-scale structured problems, this function computes the Jacobian matrix product `JY`, `J'Y`, or `J'(JY)` without actually forming `J`. The function is of the form W = jmfun(Jinfo,Y,flag) where `Jinfo` contains the matrix used to compute `JY`(or `J'Y`, or `J'(JY)`). The first argument `Jinfo` must be the same as the second argument returned by the objective function `fun`, for example, by [F,Jinfo] = fun(x) `Y` is a matrix that has the same number of rows as there are dimensions in the problem. `flag` determines which product to compute: If `flag == 0` then `W = J'(JY)`. If `flag > 0` then `W = JY`. If `flag < 0` then `W = J'Y`. In each case, `J` is not formed explicitly. The solver uses `Jinfo` to compute the preconditioner. See Passing Extra Parameters for information on how to supply values for any additional parameters `jmfun` needs. Note `'SpecifyObjectiveGradient'` must be set to `true` for the solver to pass `Jinfo` from `fun` to `jmfun`. See Minimization with Dense Structured Hessian, Linear Equalities and Jacobian Multiply Function with Linear Least Squares for similar examples. For `optimset`, the name is `JacobMult`. See Current and Legacy Option Names.
JacobPattern	Sparsity pattern of the Jacobian for finite differencing. Set `JacobPattern(i,j) = 1` when `fun(i)` depends on `x(j)`. Otherwise, set `JacobPattern(i,j) = 0`. In other words, `JacobPattern(i,j) = 1` when you can have ∂`fun(i)`/∂`x(j)` ≠ 0. Use `JacobPattern` when it is inconvenient to compute the Jacobian matrix `J` in `fun`, though you can determine (say, by inspection) when `fun(i)` depends on `x(j)`. The solver can approximate `J` via sparse finite differences when you give `JacobPattern`. If the structure is unknown, do not set `JacobPattern`. The default behavior is as if `JacobPattern` is a dense matrix of ones. Then the solver computes a full finite-difference approximation in each iteration. This can be expensive for large problems, so it is usually better to determine the sparsity structure.
MaxPCGIter	Maximum number of PCG (preconditioned conjugate gradient) iterations, a positive scalar. The default is `max(1,numberOfVariables/2)`. For more information, see Large Scale Nonlinear Least Squares.
PrecondBandWidth	Upper bandwidth of preconditioner for PCG, a nonnegative integer. The default `PrecondBandWidth` is `Inf`, which means a direct factorization (Cholesky) is used rather than the conjugate gradients (CG). The direct factorization is computationally more expensive than CG, but produces a better quality step towards the solution. Set `PrecondBandWidth` to `0` for diagonal preconditioning (upper bandwidth of 0). For some problems, an intermediate bandwidth reduces the number of PCG iterations.
`SubproblemAlgorithm`	Determines how the iteration step is calculated. The default, `'factorization'`, takes a slower but more accurate step than `'cg'`. See Trust-Region-Reflective Least Squares.
TolPCG	Termination tolerance on the PCG iteration, a positive scalar. The default is `0.1`.
Levenberg-Marquardt Algorithm
InitDamping	Initial value of the Levenberg-Marquardt parameter, a positive scalar. Default is `1e-2`. For details, see Levenberg-Marquardt Method.
ScaleProblem	`'jacobian'` can sometimes improve the convergence of a poorly scaled problem; the default is `'none'`.

Example: options = optimoptions('lsqnonlin','FiniteDifferenceType','central')

`problem` — Problem structure
structure

Problem structure, specified as a structure with the following fields:

Field Name	Entry
`objective`	Objective function
`x0`	Initial point for `x`
`lb`	Vector of lower bounds
`ub`	Vector of upper bounds
`solver`	`'lsqnonlin'`
`options`	Options created with `optimoptions`

You must supply at least the objective, x0, solver, and options fields in the problem structure.

Data Types: struct

Output Arguments

collapse all

`x` — Solution
real vector | real array

Solution, returned as a real vector or real array. The size of x is the same as the size of x0. Typically, x is a local solution to the problem when exitflag is positive. For information on the quality of the solution, see When the Solver Succeeds.

`resnorm` — Squared norm of the residual
nonnegative real

Squared norm of the residual, returned as a nonnegative real. resnorm is the squared 2-norm of the residual at x: sum(fun(x).^2).

`residual` — Value of objective function at solution
array

Value of objective function at solution, returned as an array. In general, residual = fun(x).

`exitflag` — Reason the solver stopped
integer

Reason the solver stopped, returned as an integer.

`1`	Function converged to a solution `x`.
`2`	Change in `x` is less than the specified tolerance, or Jacobian at `x` is undefined.
`3`	Change in the residual is less than the specified tolerance.
`4`	Relative magnitude of search direction is smaller than the step tolerance.
`0`	Number of iterations exceeds `options.MaxIterations` or number of function evaluations exceeded `options.MaxFunctionEvaluations`.
`-1`	A plot function or output function stopped the solver.
`-2`	Problem is infeasible: the bounds `lb` and `ub` are inconsistent.

`output` — Information about the optimization process
structure

Information about the optimization process, returned as a structure with fields:

`firstorderopt`	Measure of first-order optimality
`iterations`	Number of iterations taken
`funcCount`	The number of function evaluations
`cgiterations`	Total number of PCG iterations (trust-region-reflective algorithm only)
`stepsize`	Final displacement in `x`
`algorithm`	Optimization algorithm used
`message`	Exit message

`lambda` — Lagrange multipliers at the solution
structure

Lagrange multipliers at the solution, returned as a structure with fields:

`lower`	Lower bounds `lb`
`upper`	Upper bounds `ub`

`jacobian` — Jacobian at the solution
real matrix

Jacobian at the solution, returned as a real matrix. jacobian(i,j) is the partial derivative of fun(i) with respect to x(j) at the solution x.

Limitations

The trust-region-reflective algorithm does not solve underdetermined systems; it requires that the number of equations, i.e., the row dimension of F, be at least as great as the number of variables. In the underdetermined case, lsqnonlin uses the Levenberg-Marquardt algorithm.
lsqnonlin can solve complex-valued problems directly. Noe that bound constraints do not make sense for complex values. For a complex problem with bound constraints, split the variables into real and imaginary parts. See Fit a Model to Complex-Valued Data.
The preconditioner computation used in the preconditioned conjugate gradient part of the trust-region-reflective method forms J^TJ (where J is the Jacobian matrix) before computing the preconditioner. Therefore, a row of J with many nonzeros, which results in a nearly dense product J^TJ, can lead to a costly solution process for large problems.
If components of x have no upper (or lower) bounds, lsqnonlin prefers that the corresponding components of ub (or lb) be set to inf (or -inf for lower bounds) as opposed to an arbitrary but very large positive (or negative for lower bounds) number.

You can use the trust-region reflective algorithm in lsqnonlin, lsqcurvefit, and fsolve with small- to medium-scale problems without computing the Jacobian in fun or providing the Jacobian sparsity pattern. (This also applies to using fmincon or fminunc without computing the Hessian or supplying the Hessian sparsity pattern.) How small is small- to medium-scale? No absolute answer is available, as it depends on the amount of virtual memory in your computer system configuration.

Suppose your problem has m equations and n unknowns. If the command J = sparse(ones(m,n)) causes an Out of memory error on your machine, then this is certainly too large a problem. If it does not result in an error, the problem might still be too large. You can find out only by running it and seeing if MATLAB runs within the amount of virtual memory available on your system.

Algorithms

The Levenberg-Marquardt and trust-region-reflective methods are based on the nonlinear least-squares algorithms also used in fsolve.

The default trust-region-reflective algorithm is a subspace trust-region method and is based on the interior-reflective Newton method described in [1] and [2]. Each iteration involves the approximate solution of a large linear system using the method of preconditioned conjugate gradients (PCG). See Trust-Region-Reflective Least Squares.
The Levenberg-Marquardt method is described in references [4], [5], and [6]. See Levenberg-Marquardt Method.

Alternative Functionality

App

The Optimize Live Editor task provides a visual interface for lsqnonlin.

References

[1] Coleman, T.F. and Y. Li. “An Interior, Trust Region Approach for Nonlinear Minimization Subject to Bounds.” SIAM Journal on Optimization, Vol. 6, 1996, pp. 418–445.

[2] Coleman, T.F. and Y. Li. “On the Convergence of Reflective Newton Methods for Large-Scale Nonlinear Minimization Subject to Bounds.” Mathematical Programming, Vol. 67, Number 2, 1994, pp. 189–224.

[3] Dennis, J. E. Jr. “Nonlinear Least-Squares.” State of the Art in Numerical Analysis, ed. D. Jacobs, Academic Press, pp. 269–312.

[4] Levenberg, K. “A Method for the Solution of Certain Problems in Least-Squares.” Quarterly Applied Mathematics 2, 1944, pp. 164–168.

[5] Marquardt, D. “An Algorithm for Least-squares Estimation of Nonlinear Parameters.” SIAM Journal Applied Mathematics, Vol. 11, 1963, pp. 431–441.

[6] Moré, J. J. “The Levenberg-Marquardt Algorithm: Implementation and Theory.” Numerical Analysis, ed. G. A. Watson, Lecture Notes in Mathematics 630, Springer Verlag, 1977, pp. 105–116.

[7] Moré, J. J., B. S. Garbow, and K. E. Hillstrom. User Guide for MINPACK 1. Argonne National Laboratory, Rept. ANL–80–74, 1980.

[8] Powell, M. J. D. “A Fortran Subroutine for Solving Systems of Nonlinear Algebraic Equations.” Numerical Methods for Nonlinear Algebraic Equations, P. Rabinowitz, ed., Ch.7, 1970.

Extended Capabilities

C/C++ Code Generation
Generate C and C++ code using MATLAB® Coder™.

lsqcurvefit and lsqnonlin support code generation using either the codegen (MATLAB Coder) function or the MATLAB Coder™ app. You must have a MATLAB Coder license to generate code.
The target hardware must support standard double-precision floating-point computations. You cannot generate code for single-precision or fixed-point computations.
Code generation targets do not use the same math kernel libraries as MATLAB solvers. Therefore, code generation solutions can vary from solver solutions, especially for poorly conditioned problems.
All code for generation must be MATLAB code. In particular, you cannot use a custom black-box function as an objective function for lsqcurvefit or lsqnonlin. You can use coder.ceval to evaluate a custom function coded in C or C++. However, the custom function must be called in a MATLAB function.
lsqcurvefit and lsqnonlin do not support the problem argument for code generation.
```
[x,fval] = lsqnonlin(problem) % Not supported
```

You must specify the objective function by using function handles, not strings or character names.

x = lsqnonlin(@fun,x0,lb,ub,options) % Supported
% Not supported: lsqnonlin('fun',...) or lsqnonlin("fun",...)

All input matrices lb and ub must be full, not sparse. You can convert sparse matrices to full by using the full function.
The lb and ub arguments must have the same number of entries as the x0 argument or must be empty [].
For advanced code optimization involving embedded processors, you also need an Embedded Coder^® license.
You must include options for lsqcurvefit or lsqnonlin and specify them using optimoptions. The options must include the Algorithm option, set to 'levenberg-marquardt'.
```
options = optimoptions('lsqnonlin','Algorithm','levenberg-marquardt');
[x,fval,exitflag] = lsqnonlin(fun,x0,lb,ub,options);
```
Code generation supports these options:
- Algorithm — Must be 'levenberg-marquardt'
- FiniteDifferenceStepSize
- FiniteDifferenceType
- FunctionTolerance
- MaxFunctionEvaluations
- MaxIterations
- SpecifyObjectiveGradient
- StepTolerance
- TypicalX

Generated code has limited error checking for options. The recommended way to update an option is to use optimoptions, not dot notation.

opts = optimoptions('lsqnonlin','Algorithm','levenberg-marquardt');
opts = optimoptions(opts,'MaxIterations',1e4); % Recommended
opts.MaxIterations = 1e4; % Not recommended

Do not load options from a file. Doing so can cause code generation to fail. Instead, create options in your code.
Usually, if you specify an option that is not supported, the option is silently ignored during code generation. However, if you specify a plot function or output function by using dot notation, code generation can issue an error. For reliability, specify only supported options.
Because output functions and plot functions are not supported, solvers do not return the exit flag –1.

For an example, see Generate Code for lsqcurvefit or lsqnonlin.

Automatic Parallel Support
Accelerate code by automatically running computation in parallel using Parallel Computing Toolbox™.

To run in parallel, set the 'UseParallel' option to true.

options = optimoptions('solvername','UseParallel',true)

For more information, see Using Parallel Computing in Optimization Toolbox.

Documentation

lsqnonlin

Syntax

Description

Examples

Fit a Simple Exponential

Fit a Problem with Bound Constraints

Nonlinear Least Squares with Nondefault Options

Nonlinear Least Squares Solution and Residual Norm

Examine the Solution Process

Input Arguments

`fun` — Function whose sum of squares is minimized
function handle | name of function

`x0` — Initial point
real vector | real array

`lb` — Lower bounds
real vector | real array

`ub` — Upper bounds
real vector | real array

`options` — Optimization options
output of `optimoptions` | structure as `optimset` returns

`problem` — Problem structure
structure

Output Arguments

`x` — Solution
real vector | real array

`resnorm` — Squared norm of the residual
nonnegative real

`residual` — Value of objective function at solution
array

`exitflag` — Reason the solver stopped
integer

`output` — Information about the optimization process
structure

`lambda` — Lagrange multipliers at the solution
structure

`jacobian` — Jacobian at the solution
real matrix

Limitations

Algorithms

Alternative Functionality

App

References

Extended Capabilities

C/C++ Code Generation
Generate C and C++ code using MATLAB® Coder™.

Automatic Parallel Support
Accelerate code by automatically running computation in parallel using Parallel Computing Toolbox™.

See Also

Topics

Optimization Toolbox Documentation

Support

Documentation

lsqnonlin

Syntax

Description

Examples

Fit a Simple Exponential

Fit a Problem with Bound Constraints

Nonlinear Least Squares with Nondefault Options

Nonlinear Least Squares Solution and Residual Norm

Examine the Solution Process

Input Arguments

fun — Function whose sum of squares is minimized function handle | name of function

x0 — Initial point real vector | real array

lb — Lower bounds real vector | real array

ub — Upper bounds real vector | real array

options — Optimization options output of optimoptions | structure as optimset returns

problem — Problem structure structure

Output Arguments

x — Solution real vector | real array

resnorm — Squared norm of the residual nonnegative real

residual — Value of objective function at solution array

exitflag — Reason the solver stopped integer

output — Information about the optimization process structure

lambda — Lagrange multipliers at the solution structure

jacobian — Jacobian at the solution real matrix

Limitations

Algorithms

Alternative Functionality

App

References

Extended Capabilities

C/C++ Code Generation Generate C and C++ code using MATLAB® Coder™.

Automatic Parallel Support Accelerate code by automatically running computation in parallel using Parallel Computing Toolbox™.

See Also

Topics

Optimization Toolbox Documentation

Support

`fun` — Function whose sum of squares is minimized
function handle | name of function

`x0` — Initial point
real vector | real array

`lb` — Lower bounds
real vector | real array

`ub` — Upper bounds
real vector | real array

`options` — Optimization options
output of `optimoptions` | structure as `optimset` returns

`problem` — Problem structure
structure

`x` — Solution
real vector | real array

`resnorm` — Squared norm of the residual
nonnegative real

`residual` — Value of objective function at solution
array

`exitflag` — Reason the solver stopped
integer

`output` — Information about the optimization process
structure

`lambda` — Lagrange multipliers at the solution
structure

`jacobian` — Jacobian at the solution
real matrix

C/C++ Code Generation
Generate C and C++ code using MATLAB® Coder™.

Automatic Parallel Support
Accelerate code by automatically running computation in parallel using Parallel Computing Toolbox™.