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A158914 Primes p such that there is a composite c with sigma_2(p)=sigma_2(c). 1
7, 47, 157, 3863 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

No other terms less than 10^8. The corresponding composite numbers are 6, 40, 136, and 3352. Is this sequence finite? See A158913 for the sequence for sigma_1.

LINKS

Table of n, a(n) for n=1..4.

MATHEMATICA

tp=DivisorSigma[2, Select[Range[4000], PrimeQ]]; tc=DivisorSigma[2, Select[Range[4000], !PrimeQ[ # ]&]]; Sqrt[Intersection[tp, tc]-1]

CROSSREFS

Sequence in context: A009219 A158713 A142185 * A046872 A167860 A152988

Adjacent sequences:  A158911 A158912 A158913 * A158915 A158916 A158917

KEYWORD

more,nonn

AUTHOR

T. D. Noe, Mar 30 2009

STATUS

approved

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Last modified April 12 17:26 EDT 2021. Contains 342929 sequences. (Running on oeis4.)