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A107914 Numbers k that require four iterations of the sigma function to be >= 2*k. 4
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 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
Most of the early terms are the square of a prime or the fourth power of a prime.
LINKS
EXAMPLE
sigma(sigma(sigma(81))) = 160 but sigma(sigma(sigma(sigma(81)))) = 378, so 81 is in the sequence.
PROG
(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
CROSSREFS
Sequence in context: A297681 A237404 A237549 * A050634 A065791 A237503
KEYWORD
nonn
AUTHOR
Jud McCranie, May 27 2005
EXTENSIONS
Offset 1 by Michel Marcus, Apr 25 2020
More terms from Amiram Eldar, Mar 18 2024
STATUS
approved

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Last modified July 17 18:40 EDT 2024. Contains 374377 sequences. (Running on oeis4.)