A107914 Numbers k that require four iterations of the sigma function to be >= 2*k.

81, 1681, 3481, 5041, 7921, 17161, 27889, 29929, 83521, 146689, 279841, 491401, 552049, 579121, 635209, 683929, 703921, 707281, 829921, 1190281, 1203409, 1352569, 1481089, 1885129, 2036329, 2211169, 2430481, 2505889, 3279721, 3411409, 3523129, 3568321, 3728761

Offset: 1

Author

Jud McCranie, May 27 2005

Keywords

• nonn

Comments

Most of the early terms are the square of a prime or the fourth power of a prime.

Examples

			sigma(sigma(sigma(81))) = 160 but sigma(sigma(sigma(sigma(81)))) = 378, so 81 is in the sequence.
		

Links

• Amiram Eldar, Table of n, a(n) for n = 1..500

Crossrefs

Cf. A000203, A055020, A055021, A107912, A107913.

Programs

PARI

is(n) = {my(m = n, nn = 2*n, c = 0); while(m < nn, m = sigma(m); c++); c == 4;} \\ Amiram Eldar, Mar 18 2024

Extensions

Offset 1 by Michel Marcus, Apr 25 2020

More terms from Amiram Eldar, Mar 18 2024