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 A115905 #15 Apr 06 2023 08:30:50 %S A115905 2,4,9,11,20,23,25,27,49,53,92,93,105,121,127,147,150,163,169,220,228, %T A115905 231,233,281,286,289,294,332,361,431,474,529,541,543,548,613,660,675, %U A115905 710,726,743,825,827,841,888,891,961,977,993,1014,1028,1265,1310,1369 %N A115905 Numbers k such that k + phi(k) is a triangular number. %H A115905 Amiram Eldar, <a href="/A115905/b115905.txt">Table of n, a(n) for n = 1..10000</a> %e A115905 121 + phi(121) = 231 = T(21). %t A115905 Select[Range[1500],OddQ[Sqrt[8(#+EulerPhi[#])+1]]&] (* _Harvey P. Dale_, May 11 2015 *) %o A115905 (PARI) isok(n) = ispolygonal(n + eulerphi(n), 3); \\ _Michel Marcus_, Jan 25 2014 %Y A115905 Cf. A000010, A000217, A069066, A115908, A115910. %K A115905 nonn %O A115905 1,1 %A A115905 _Giovanni Resta_, Feb 06 2006