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%I #21 Mar 09 2024 11:01:39
%S 1,4,5,50,64,906,966,5805,40514,133667,262277,1416109,42142704,
%T 189758142,350476553,957982453,1420733777,1421477786,2557347701,
%U 28609375750,95023678204,100094778026,119964793932
%N Numbers k such that Sum_{j=1..k} sigma(j) is a triangular number, where sigma(j) = sum of divisors of j (A000203).
%C Indices of triangular numbers in A024916.
%C The sequence of indices of generated triangular numbers b(n) begins: 1, 5, 6, 64, 82, 1162, 1239, 7445, 51961, 171434, 336383, 1816230, 54050118, 243374273, 449503643, 1228660232, 1822161864, 1823116093, 3279925859. A024916(a(n)) = A000217(b(n)).
%H Lucas A. Brown, <a href="https://github.com/lucasaugustus/oeis/blob/main/A226648.py">Python program</a>.
%o (PARI) isok(n) = ispolygonal(sum(k=1, n, sigma(k)), 3); \\ _Michel Marcus_, Nov 08 2014
%o (Python) # See LINKS.
%Y Cf. A000203, A000217, A024916, A130698.
%K nonn,more,hard
%O 1,2
%A _Alex Ratushnyak_, Jun 13 2013
%E a(20)-a(23) from _Lucas A. Brown_, Mar 08 2024
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