* Problem 3 should read:

  "Show that the Wronskian of two fundamental sets of solutions of
  x'=P(t)x [can differ at most by a multiplicative constant]".

* Problem 5 solution should be corrected as follows:

  (Let p be the particular solution and let x the other solution.)
  NOT: (x-p)' = x'-p' = g-g = 0
  BUT: (x-p)' = x'-p' = (Px+g) - P(p+g) = P(x-p) = 0

The following corrections were made from the first posting:

* Problem 6 part b solution now reads:

  "x^(1) and x^(2) are L.I. at every point except at t=0;
  they are linearly independent on every interval.

* Problem 6 part d is now solved systematically and completely.