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%I #16 Oct 15 2016 08:54:04
%S 0,1,4,5,9,19,148,209,404,652,763,1739,2790,9551,11117,13173,25641,
%T 170003,181927,519900,3253342,4816257,7103608,8269588,64261168,
%U 177847445,294878941,819667577,844965349,2944185248
%N Numbers n such that A002088(n) is a triangular number.
%C Corresponding triangular numbers are 0, 1, 6, 10, 28, 120, 6670, 13366, 49770, 129286, 177310, ...
%e 9 is a term because A002088(9) = 1+1+2+2+4+2+6+4+6 = 28 = 1+2+3+4+5+6+7.
%o (PARI) lista(nn) = {print1(st = 0, ", "); for(n=1, nn, st += eulerphi(n); if (ispolygonal(st, 3), print1(n, ", ")));} \\ _Michel Marcus_, Sep 09 2016
%Y Cf. A000217, A002088.
%K nonn,more
%O 1,3
%A _Altug Alkan_, Sep 09 2016
%E More terms from _Michel Marcus_, Sep 09 2016
%E a(25)-a(30) from _Lars Blomberg_, Oct 15 2016
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