The map that takes a quaternion x to the quaternion qx is linear, represented as multiplication by a matrix.
The matrix representation for multiplying on the left by q is different from the matrix representation for multiplying on the right by q.
One can prove both matrix representations are correct by demonstrating their accuracy when q = 1, i, j, and k.
The post discusses how to represent rotations with quaternions and provides an equation for the equivalent rotation matrix for a rotation described by a quaternion.