Hi I have a stream cipher which uses a keystream of:

F(r) || F(r+1) || F(r+2) || ...

Suppose F(r) is the RSA encryption algorithm:

F(r) = r^e mod p where e and p are known , e = 3

My question is: If we know F(r), F(r+1) and F(r+2), how can we calculate the subsequent key sequence?

Is there some obvious relation between F(r+3) and F(r),F(r+1),F(r+2) I don't see?