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 A287472 #12 May 28 2017 09:33:55 %S A287472 1,231,1035,6786,190036,193131,766941,1237951,1348903,3069003,3396921, %T A287472 8034036,9152781,11875501,15694003,28001386,29587278,35149920, %U A287472 61643856,63196903,130758706,178161126,198214005,227751153,268111746,339210081,402102261,654224878 %N A287472 Triangular numbers k such that phi(k) is also a triangular number, where phi(k) is the Euler totient function (A000010). %C A287472 The indices of these triangular numbers are: 1, 21, 45, 116, 616, 621, 1238, 1573, 1642, 2477, 2606, 4008, 4278, 4873, 5602, 7483, 7692, 8384, 11103, 11242, 16171, 18876, 19910, 21342, 23156, 26046, 28358, 36172, 46196, 46621, 67572, 72816, ... %C A287472 The indices of the triangular phi values are: 1, 15, 32, 63, 384, 495, 927, 1440, 1599, 1856, 2015, 2240, 3200, 4640, 5375, 4895, 4095, 4095, 6400, 9855, 10880, 9855, 13824, 16128, 12095, 19520, 21504, 25344, 25983, 45584, 37184, 40959, ... %H A287472 Amiram Eldar, <a href="/A287472/b287472.txt">Table of n, a(n) for n = 1..100</a> %e A287472 231 = 21*22/2 is triangular, phi(231)=120=15*16/2 is also triangular, thus 231 is in the sequence. %t A287472 triQ[n_] := IntegerQ@Sqrt[8n+1]; Select[Accumulate[Range[1000]], triQ[EulerPhi[#]]&] %o A287472 (PARI) isok(n) = ispolygonal(n, 3) && ispolygonal(eulerphi(n), 3); \\ _Michel Marcus_, May 25 2017 %Y A287472 Cf. A000010, A000217, A083674, A083675, A115910, A116541. %K A287472 nonn %O A287472 1,2 %A A287472 _Amiram Eldar_, May 25 2017