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A065997 Numbers n such that sigma(n) / n is prime. 11
6, 28, 120, 496, 672, 8128, 523776, 33550336, 459818240, 1476304896, 8589869056, 14182439040, 31998395520, 51001180160, 137438691328, 518666803200, 13661860101120, 30823866178560, 740344994887680, 796928461056000, 212517062615531520, 2305843008139952128 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
This is a subsequence of the sequence of multiply perfect numbers A007691.
The prime values of sigma(n) / n are A219545.
Numbers whose abundancy index is a prime. There are two visible bends (sudden changes in the growth rate) in the scatter plot. Compare also to the scatter plot of A336702. - Antti Karttunen, Feb 25 2022
LINKS
Antti Karttunen, Table of n, a(n) for n = 1..597 (computed from the b-file of A007691 prepared by T. D. Noe, using Flammenkamp's data)
PROG
(PARI) isA065997(n) = { my(p=sigma(n)/n); (1==denominator(p) && isprime(p)); }; \\ Antti Karttunen, Feb 25 2022
CROSSREFS
Subsequence of A007691 and of A342924.
Cf. A000396, A005820, A046060 (subsequences).
Sequence in context: A348031 A260508 A334410 * A348035 A354073 A171476
KEYWORD
nonn,look
AUTHOR
Joseph L. Pe, Dec 10 2001
EXTENSIONS
Terms a(10) to a(14) from Jonathan Sondow, Nov 22 2012
Extended by T. D. Noe, Nov 26 2012
STATUS
approved

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Last modified March 18 22:56 EDT 2024. Contains 370952 sequences. (Running on oeis4.)