replace (p * n) with (n * p). trivial with arith. apply mult_sym.
Qed.
(****************************************************************)
(* La fonction puissance de 2 *)
Fixpoint power2 (n : nat) : nat :=
match n with
| O => 1
| S x => power2 x + power2 x
end.

Lemma power2_eq2 : forall x : nat, power2 (S x) = power2 x + power2 x.