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 A115910 #21 Apr 06 2023 08:30:39 %S A115910 1,3,15,84,217,255,1710,2967,5363,11242,24066,27370,29697,29953,60977, %T A115910 65535,69324,103257,159761,209305,228333,446516,598559,615410,691410, %U A115910 1231305,1238358,1365175,1467732,1841154,1966220,2081070,2562370 %N A115910 Numbers k such that phi(k)*k is a triangular number. %H A115910 Amiram Eldar, <a href="/A115910/b115910.txt">Table of n, a(n) for n = 1..100</a> %e A115910 phi(24066)*24066 = 164611440 = T(18144). %t A115910 Select[Range[2.6*10^6],OddQ[Sqrt[8#*EulerPhi[#]+1]]&] (* _Harvey P. Dale_, Jul 11 2017 *) %o A115910 (PARI) isok(n) = ispolygonal(n*eulerphi(n), 3); \\ _Michel Marcus_, Jan 25 2014 %Y A115910 Cf. A002618, A000217, A115905, A115909. %K A115910 nonn %O A115910 1,2 %A A115910 _Giovanni Resta_, Feb 06 2006