This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A278509 #11 Aug 13 2023 02:17:24
%S A278509 1,1,3,1,1,3,3,1,9,1,3,3,1,3,3,1,1,9,3,1,9,3,3,3,1,1,27,3,1,3,3,1,9,1,
%T A278509 3,9,1,3,3,1,1,9,3,3,9,3,3,3,9,1,3,1,1,27,3,3,9,1,3,3,1,3,27,1,1,9,3,
%U A278509 1,9,3,3,9,1,1,3,3,9,3,3,1,81,1,3,9,1,3,3,3,1,9,3,3,9,3,3,3,1,9,27,1,1,3,3,1,9,1,3,27,1,3,3,3,1,9,3,1,9,3,3,3
%N A278509 a(n) = 3^{number of primes congruent to 3 modulo 4 dividing n (with multiplicity)}.
%H A278509 Antti Karttunen, <a href="/A278509/b278509.txt">Table of n, a(n) for n = 1..10000</a>
%F A278509 Fully multiplicative with a(p^e) = 1 if p = 2, (p mod 4)^e if p > 2.
%F A278509 a(n) = A065338(A000265(n)) = A000265(A065338(n)).
%F A278509 a(n) = A000244(A065339(n)) = 3^A065339(n).
%t A278509 f[p_, e_] := Mod[p, 4]^e; f[2, e_] := 1; a[1] = 1; a[n_] := Times @@ f @@@ FactorInteger[n]; Array[a, 100] (* _Amiram Eldar_, Aug 13 2023 *)
%o A278509 (Scheme) (define (A278509 n) (A065338 (A000265 n)))
%Y A278509 Cf. A000244, A000265, A065338, A065339.
%Y A278509 Cf. also A278265.
%K A278509 nonn,mult
%O A278509 1,3
%A A278509 _Antti Karttunen_, Nov 28 2016