What's the name of this movie about a guy who thinks he's from space but really he's crazy?

In one part he does some really impressive math equations to prove
he's an alien from space... He's in a mental hospital being evaluated
or something because they think he's crazy and it turns out he is just
crazy because his family was murdered...

Sorry it's kinda vague, I didn't see the whole movie and it was a
while ago, but hopefully someone knows what I'm talking about!