|
%I #12 Jul 22 2021 01:57:16
%S 1,2,22,3051,3219,3393,5057,8653,75618,95675,100503,102949,104714,
%T 287826,438547,522339,537159,688050,2191200,2317118,2418548,2507683,
%U 2599128,3212964,4534573,5367797,6047913,6302639,7689149,13758296,14380145,15342050,16148979
%N Numbers k such that sigma(k) and phi(k) are both triangular numbers.
%C phi(k) = A000010(k) is the Euler totient function, while sigma(k) = A000203(k) is the sum of divisors of k.
%H Donovan Johnson, <a href="/A113930/b113930.txt">Table of n, a(n) for n = 1..207</a> (terms < 10^12)
%e sigma(100503) = 156520 = T(559) and phi(100503) = 61776 = T(351).
%o (PARI) isok(n) = ispolygonal(sigma(n), 3) && ispolygonal(eulerphi(n), 3); \\ _Michel Marcus_, Jan 26 2014
%Y Cf. A000010, A000203, A000217.
%Y Subsequence of A045746.
%K nonn
%O 1,2
%A _Giovanni Resta_, Jan 30 2006
|
