%% Graphical Comparison of Exponential Functions
% This example shows an interesting graphical approach for discovering whether {\mathrm{e}}^{\pi } is greater than \pi^e.
%
% The question is: which is greater, {\mathrm{e}}^{\pi } or \pi^e? The easy way to find out is to type it directly at the MATLAB® command prompt. But another way to analyze the situation is to ask a more general question: what is the shape of the function z\left(x,y\right)=x^y -y^x?
% Here is a plot of z.

% Define the mesh
x = 0:0.16:5;
y = 0:0.16:5;
[xx,yy] = meshgrid(x,y);

% The plot
zz = xx.^yy-yy.^xx;
h = surf(x,y,zz);
h.EdgeColor = [0.7 0.7 0.7];
view(20,50);
colormap(hsv);
title('$z = x^y-y^x$','Interpreter','latex')
xlabel('x')
ylabel('y')
hold on

%
% The solution of the equation x^y -y^x =0 has a very interesting shape, and our original question is not easily solved by inspection. Here is a plot of the xy values that yield z=0.

c = contourc(x,y,zz,[0 0]);
list1Len = c(2,1);
xContour = [c(1,2:1+list1Len) NaN c(1,3+list1Len:size(c,2))];
yContour = [c(2,2:1+list1Len) NaN c(2,3+list1Len:size(c,2))];
% Note that the NAN above prevents the end of the first contour line from being
% connected to the beginning of the second line
line(xContour,yContour,'Color','k');

%
% Some combinations of x and y along the black curve are integers. This next plot is of the integer solutions to the equation x^y -y^x =0. Notice that 2^4 =4^2 is the only integer solution where x\not= y.

plot([0:5 2 4],[0:5 4 2],'r.','MarkerSize',25);

%
% Finally, plot the points \left(\pi ,e\right) and \left(e,\pi \right) on the surface. The result shows that {\mathrm{e}}^{\pi } is indeed larger than \pi^e (though not by much).

e = exp(1);
plot([e pi],[pi e],'r.','MarkerSize',25);
plot([e pi],[pi e],'y.','MarkerSize',10);
text(e,3.3,'(e,pi)','Color','k', ...
   'HorizontalAlignment','left','VerticalAlignment','bottom');
text(3.3,e,'(pi,e)','Color','k','HorizontalAlignment','left',...
   'VerticalAlignment','bottom');
hold off;

%
% Verify the results.

e = exp(1);
e^pi
pi^e

%
% Copyright 1984-2014 The MathWorks, Inc.