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A236387 Numbers n such that sigma(n) is an oblong number. 2
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 (list; graph; refs; listen; history; text; internal format)
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 *)

CROSSREFS

Cf. A000203, A002327, A002378.

Sequence in context: A041533 A042881 A041052 * A039918 A020685 A275492

Adjacent sequences:  A236384 A236385 A236386 * A236388 A236389 A236390

KEYWORD

nonn,easy

AUTHOR

Joseph L. Pe, Jan 24 2014

EXTENSIONS

a(12)-a(56) from Giovanni Resta, Jan 24 2014

STATUS

approved

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Last modified November 19 22:34 EST 2019. Contains 329323 sequences. (Running on oeis4.)