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

Author

Joseph L. Pe, Jan 24 2014

Keywords

• nonn
• easy

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

Examples

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

Links

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

Crossrefs

Cf. A000203, A002327, A002378.

Programs

Mathematica

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

Extensions

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