Solve differential equation with undetermined constants making the assumption that the constants are positive

I want to solve:

eqn = D[f[w], {w,2}] + (3/2 c^2 k Exp[c w] - 9/4 c^2  k^2  Exp[2 c w]) f[w] +  m^2 f[w]

sol = DSolve[eqn == 0, f[w], w]


with the assumption that m, c and k are positive.

How do I add this assumptions?

-
this FullSimplify[f[w] /. DSolve[{eqn == 0 }, f[w], w][[1]], c >= 0 && m >= 0 && k > 0]? –  Rorschach Sep 9 '13 at 18:44
{ FullSimplify[f[w] /. DSolve[{eqn == 0 }, f[w], w][[1]], c >= 0 && m >= 0 && k > 0]} === FullSimplify[f[w] /. DSolve[{eqn == 0 }, f[w], w]] returns True..so it seems that these constant's sign have no effect. –  Rorschach Sep 9 '13 at 18:58