(** log2 is exact on powers of 2 *)

(** Log2 and multiplication. *)
(** Due to rounding error, we don't have the usual [log2 (a*b) = log2 a + log2 b] but we may be off by 1 at most *)
Lemma log2_mul_below : forall a b, 0<a -> 0<b ->
log2 a + log2 b <= log2 (a*b).