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%I #12 Jul 15 2021 21:24:20
%S 7,10,66,136,145,196,198,234,270,438,495,528,592,606,855,910,1045,
%T 1158,1428,2208,2718,2922,3016,3663,3808,3916,4506,4975,5043,5136,
%U 5598,6366,6600,7080,7498,7821,8172,8200,8217,8814,9318,9730,9738
%N Numbers k such that k + sigma(k) is a triangular number.
%H Harvey P. Dale, <a href="/A115904/b115904.txt">Table of n, a(n) for n = 1..1000</a>
%e 234 + sigma(234) = 780 = T(39).
%t trnoQ[n_]:=IntegerQ[(Sqrt[1+8n]-1)/2]; Select[Range[10000], trnoQ[#+ DivisorSigma[ 1,#]]&] (* _Harvey P. Dale_, Apr 26 2014 *)
%o (PARI) isok(n) = ispolygonal(n + sigma(n), 3); \\ _Michel Marcus_, Jan 25 2014
%Y Cf. A069066, A115907, A115909.
%K nonn
%O 1,1
%A _Giovanni Resta_, Feb 06 2006
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