A = [  3.3  6.2  7.1  9.1;
  -2.3  3.6  0.3  9.7;
  6.7  -0.1  1.1  4.8;
  8.4  7.7  5.9  -1.8;
]
B = [  1.1  2.2  3.3  4.4;
  8.8  7.7  6.6  5.5;
  6.1  8.2  9.7  3.6;
  6.3  2.9  5.5  8.1;
]

C = A * B
  158.830  139.610  170.730  147.890
  92.090  53.250  72.430  89.330
  43.440  36.910  58.520  71.770
  101.650  120.930  125.870  85.970

C * B
  3376.441  3253.290  3915.041  3279.244
  1574.501  1465.606  1849.233  1682.392
  1181.715  1067.772  1349.337  1186.150
  2485.417  2436.238  2827.357  2261.864

D = A' * A
  131.630  76.190  79.670  24.760
  76.190  110.700  90.420  77.000
  79.670  90.420  86.520  62.180
  24.760  77.000  62.180  203.180

inverse(D)
   0.020   0.006  -0.027   0.004
   0.006   0.067  -0.073  -0.004
  -0.027  -0.073   0.116  -0.005
   0.004  -0.004  -0.005   0.007

[v,d] = eig(D);

diag(d)
     5.781  35.665  149.952  340.633
v'
   0.166   0.566  -0.807   0.013
   0.662  -0.622  -0.295   0.297
   0.589   0.159   0.220  -0.761
   0.433   0.518   0.461   0.576

v * v'
   1.000   0.000   0.000   0.000
   0.000   1.000   0.000   0.000
   0.000   0.000   1.000   0.000
   0.000   0.000   0.000   1.000

sqrtm(D)
  10.390   3.048   3.783   0.293
   3.048   8.333   4.828   2.943
   3.783   4.828   6.603   2.302
   0.293   2.943   2.302  13.753

sqrtm(D) * sqrtm(D)
  131.630  76.190  79.670  24.760
  76.190  110.700  90.420  77.000
  79.670  90.420  86.520  62.180
  24.760  77.000  62.180  203.180