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 A232619 #21 Feb 22 2024 02:13:30 %S A232619 1,3199808,36210072,48138616,106449517,109040509,109903789,110003837, %T A232619 204102760401,3050610977780,3655577208308,3660654715316,3660725004788, %U A232619 5607890017600,6123048180800,7412048520512,7471435008064,53218856249194,53221469311786,71505497845764,103285924642372,176553289677888,176596975245888,176600346573888 %N A232619 Numbers k such that sigma(k) and sigma(2*k) are triangular numbers. %e A232619 sigma(3199808) = 6784086 = triangular(3683), sigma(3199808*2) = 13621590 = triangular(5219), so 3199808 is in the sequence. %o A232619 (PARI) is(n)=issquare(8*sigma(n)+1)&&issquare(8*sigma(2*n)+1) \\ _Ralf Stephan_, Nov 30 2013 %Y A232619 A subsequence of A045746. %Y A232619 Cf. A000203, A000217, A232445. %K A232619 nonn,hard %O A232619 1,2 %A A232619 _Alex Ratushnyak_, Nov 27 2013 %E A232619 a(9) from _Hiroaki Yamanouchi_, Oct 02 2014 %E A232619 Terms a(10) onward from _Max Alekseyev_, Feb 21 2024