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

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 * A344555 A039918 A020685

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