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