(** And we can't find better approximations in general. - The lower bound is exact for powers of 2. - Concerning the upper bound, for any c>1, take a=b=2^c-1, then log2 (a*b) = c+c -1 while (log2 a) = (log2 b) = c-1 *)
(** At least, we get back the usual equation when we multiply by 2 (or 2^k) *)

(** The sum of two log2 is less than twice the log2 of the sum. The large inequality is obvious thanks to monotonicity. The strict one requires some more work. This is almost a convexity inequality for points [2a], [2b] and their middle [a+b] : ideally, we would have [2*log(a+b) >= log(2a)+log(2b) = 2+log a+log b]. Here, we cannot do better: consider for instance a=2 b=4, then 1+2<2*2 *)
Lemma add_log2_lt : forall a b, 0<a -> 0<b ->
log2 a + log2 b < 2 * log2 (a+b).