m = diag([1 2 3 4])

m =

     1     0     0     0
     0     2     0     0
     0     0     3     0
     0     0     0     4

inv(m)

ans =

    1.0000         0         0         0
         0    0.5000         0         0
         0         0    0.3333         0
         0         0         0    0.2500

diary off