use v[4][4] initialization for Mat A which appears in Figure 2.3

[Ed Bueler]

Note j[4] is initialized already. It is much clearer code if I do

double v[4][4] = {{1.0, 2.0, 3.0, 0.0},
                  {2.0, 1.0, -2.0, -3.0},
...