A236387 - OEIS

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A236387

Numbers n such that sigma(n) is an oblong number.

5, 6, 11, 19, 20, 26, 28, 29, 30, 39, 40, 41, 46, 51, 55, 58, 71, 86, 89, 99, 104, 109, 114, 116, 117, 125, 131, 135, 158, 177, 181, 201, 202, 203, 209, 216, 226, 236, 239, 245, 271, 278, 306, 336, 340, 352, 377, 379, 398, 410, 411, 419, 428, 442, 447, 461

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OFFSET

1,1

COMMENTS

An oblong number (A002378) is of the form k(k+1) where k is a natural number.

The subsequence of prime terms is A002327 (primes of form n^2 - n - 1). - Michel Marcus, Jan 09 2015

LINKS

Giovanni Resta, Table of n, a(n) for n = 1..10000

EXAMPLE

sigma(40) = 90 = 9*10, an oblong number; so 40 is a term of the sequence.

MATHEMATICA

Select[Range[500], IntegerQ@ Sqrt[1+4*DivisorSigma[1, #]] &] (* Giovanni Resta, Jan 24 2014 *)