Theorem 7.21: (old 7.21)
             strictly DD
             Jacobi: exercise 14
             GS: ref (and slides)        

Theorem 7.22: (old 7.22)
             Stein-Rosenberg (skip)

Theorem 7.24: (old 7.24)
             Kahan exercise 9.

Theorem 7.25: (old 7.24)
             Ostrowski-Reich
             If A = A^t > 0 and 0 < \omega < 2, then SOR converges (skip)

Theorem 7.26: (old 7.26)
             best \omega for tridiagonal (skip?)