thx . i changed code to this (i gave to r , H their values)
<pre>NDSolve[{p'[t] + r*p[t] == r*MatrixExp[-I*H/r].p[t].MatrixExp[I*H/r],
p[0] == H}, p[t], {t, 0, 10}]
and it gave me this error "NDSolve::deqn: Equation or list of equations expected instead of True in the first argument "
