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 A232619 Numbers n such that sigma(n) and sigma(2*n) are triangular numbers. 0
 1, 3199808, 36210072, 48138616, 106449517, 109040509, 109903789, 110003837, 204102760401 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS A subsequence of A045746. Sigma(n) = A000203(n) = sum of divisors of n. LINKS EXAMPLE sigma(3199808) = 6784086 = triangular(3683),  sigma(3199808*2) = 13621590 = triangular(5219), so 3199808 is in the sequence. PROG (PARI) is(n)=issquare(8*sigma(n)+1)&&issquare(8*sigma(2*n)+1) \\ Ralf Stephan, Nov 30 2013 CROSSREFS Cf. A000203, A000217, A045746, A232445. Sequence in context: A122034 A153750 A234689 * A223287 A223484 A029820 Adjacent sequences:  A232616 A232617 A232618 * A232620 A232621 A232622 KEYWORD nonn,hard,more AUTHOR Alex Ratushnyak, Nov 27 2013 EXTENSIONS a(9) from Hiroaki Yamanouchi, Oct 02 2014 STATUS approved

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Last modified December 5 08:36 EST 2021. Contains 349543 sequences. (Running on oeis4.)