import sympy as sp
print('sympy version:', sp.__version__)

t = sp.symbols('t', real=True, nonnegative=True)
n = sp.symbols('n', integer=True, nonnegative=True)
f = sp.symbols('f', cls=sp.Function)

diff_eq = sp.Eq(f(t).diff(t, 2) + n**2*f(t), 0)

print('general solution:', sp.dsolve(diff_eq, f(t)))
print('solution at n=0 (pre-subs):', sp.dsolve(diff_eq.subs(n, 0), f(t)))
print('solution at n=0 (post-subs):', sp.dsolve(diff_eq, f(t)).subs(n, 0))