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 A115911 #12 Jul 15 2021 21:24:55 %S A115911 1,2,9,11,23,118,373,556,1332,2420,3081,5251,7642,12671,116836,127627, %T A115911 135861,172676,198912,365408,421902,426710,901273,921736,954068, %U A115911 1001396,1003333,1006567,1077452,1161754,1162514,1364225,1632361 %N A115911 Numbers k such that phi(k)*sigma(k) is a triangular number. %H A115911 Amiram Eldar, <a href="/A115911/b115911.txt">Table of n, a(n) for n = 1..160</a> %e A115911 phi(1003333)*sigma(1003333) = 1003888529280 = T(1416960). %t A115911 Select[Range[10^4], IntegerQ @ Sqrt[8 * EulerPhi[#] * DivisorSigma[1, #] + 1] &] (* _Amiram Eldar_, Sep 16 2019 *) %o A115911 (PARI) isok(n) = ispolygonal(eulerphi(n)*sigma(n), 3); \\ _Michel Marcus_, Jan 09 2014 %Y A115911 Cf. A062354, A069066. %K A115911 nonn %O A115911 1,2 %A A115911 _Giovanni Resta_, Feb 06 2006