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 A371640 #10 Mar 31 2024 11:41:02 %S A371640 9,81,2187,6561,59049,1594323,4782969,43046721,3486784401,3486784401, %T A371640 31381059609,847288609443,2541865828329,22876792454961, %U A371640 617673396283947,1853020188851841,16677181699666569,1350851717672992089,1350851717672992089,12157665459056928801,328256967394537077627 %N A371640 a(n) = 3^(2*n + valuation(n, 3)) = 3^A371638(n). %C A371640 See A371639 for the connection with Voronoi's congruence. %F A371640 a(n) = denominator(Voronoi(3, 2*n)) where Voronoi(c, n) = ((c^n - 1)*Bernoulli(n)) / (n*c^(n - 1)). %p A371640 A371640 := n -> 3^(2*n + padic:-ordp(n, 3)): %p A371640 seq(A371640(n), n = 1..21); %o A371640 (SageMath) %o A371640 def A371640(n): return 3**(2*n + valuation(n, 3)) %o A371640 print([A371640(n) for n in range(1, 22)]) %Y A371640 Cf. A371638, A371639 (numerator Voronoi). %K A371640 nonn %O A371640 1,1 %A A371640 _Peter Luschny_, Mar 30 2024