Am I missing something obvious? I don't see in OEIS: Product of odd divisors of n 1, 1, 3, 1, 5, 3, 7, 1, 27, 5, 11, 3, 13, 7, 225, 1, 17, 27, 19, 5, 441, 11, 23, 3, 125, ... Cf. A000265, A000593, A007955, A007956, A078701 a(p) = p, if p noncomposite a(2^n) = 1. a(pq) = p^2 * q^2 when p, q both odd primes