Quaternion subtraction is defined as the subtraction of the corresponding parts of each quaternion. Create two quaternions and perform subtraction.

Q1 = quaternion([1,0,-2,7]);
Q2 = quaternion([1,2,3,4]);

Q1minusQ2 = Q1 - Q2

Q1minusQ2 = quaternion
     0 - 2i - 5j + 3k

Addition and subtraction of real numbers is defined for quaternions as acting on the real part of the quaternion. Create a quaternion and then subtract 1 from the real part.

Q = quaternion([1,1,1,1])

Q = quaternion
     1 + 1i + 1j + 1k

Qminus1 = Q - 1

Qminus1 = quaternion
     0 + 1i + 1j + 1k