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A330598
Numbers k such that the denominator of sigma(sigma(k))/k is equal to 2.
5
30, 2046, 245760, 301056, 450560, 1171456, 1351680, 3514368, 14515200, 16760832, 19611648, 77220864, 159373824, 357291648, 391444480, 477216768, 555714432, 754928640, 765414240, 1006602240, 1761500160, 2330913312, 4314834944, 8369053056, 20449394784, 37949317120
OFFSET
1,1
COMMENTS
Although the definition here is similar to the one in A019278, it appears that this sequence does not have the same nice features as A019278.
Otherwise said: sigma(sigma(k))/k is half-integer, or: sigma(sigma(k)) is an odd multiple of k/2. This also implies that all terms are even. - M. F. Hasler, Jan 06 2020
LINKS
Giovanni Resta, Table of n, a(n) for n = 1..38 (terms < 10^13)
EXAMPLE
sigma(sigma(30))/30 = sigma(72)/30 = 195/30 = 13/2 so 30 is a term.
PROG
(PARI) isok(n) = denominator(sigma(sigma(n))/n) == 2;
CROSSREFS
Cf. A019278 (denominator is 1), A051027 (sigma(sigma)).
Cf. A000203 (sigma), A159907 (hemiperfect numbers).
Sequence in context: A280670 A092608 A241128 * A255958 A092617 A295446
KEYWORD
nonn
AUTHOR
Michel Marcus, Dec 19 2019
EXTENSIONS
a(22)-a(26) from Giovanni Resta, Dec 20 2019
STATUS
approved