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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

Table of n, a(n) for n=1..9.

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.)