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 A287473 #13 May 28 2017 09:34:03 %S A287473 1,10,136,630,2016,7875,9180,18915,32896,37128,46056,58311,66430, %T A287473 103740,131841,198135,225456,301476,323610,332520,408156,499500, %U A287473 738720,786885,839160,862641,922761,924120,1065070,1079715,1183491,1385280,1851850,1906128,1925703 %N A287473 Triangular numbers k such that phi(k) is a square number, where phi(k) is the Euler totient function (A000010). %C A287473 The indices of these triangular numbers are: 1, 4, 16, 35, 63, 125, 135, 194, 256, 272, 303, 341, 364, 455, 513, 629, 671, 776, 804, 815, 903, 999, 1215, 1254, 1295, 1313, 1358, 1359, 1459, 1469, 1538, 1664, 1924, 1952, 1962, ... and their phi values are the squares of: 1, 2, 8, 12, 24, 60, 48, 96, 128, 96, 120, 180, 144, 144, 288, 288, 240, 288, 264, 288, 336, 360, 432, 600, 432, 720, 720, 480, 648, 672, 864, 576, 720, 720, 1080, ... %C A287473 Similar to A115910, since A115910(n)^2 are squares whose phi is a triangular number. %H A287473 Amiram Eldar, <a href="/A287473/b287473.txt">Table of n, a(n) for n = 1..1000</a> %e A287473 136=16*17/2 is triangular, phi(136)=64=8^2 is a square, thus 136 is in the sequence. %t A287473 Select[Accumulate[Range[1000]],IntegerQ[Sqrt[EulerPhi[#]]]&] %o A287473 (PARI) isok(n) = ispolygonal(n, 3) && issquare(eulerphi(n)); \\ _Michel Marcus_, May 25 2017 %Y A287473 Cf. A000010, A000217, A000290, A039770, A115910, A256151. %Y A287473 Intersection of A000217 and A039770. %K A287473 nonn %O A287473 1,2 %A A287473 _Amiram Eldar_, May 25 2017