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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
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.
MATHEMATICA
tp=DivisorSigma[2, Select[Range[4000], PrimeQ]]; tc=DivisorSigma[2, Select[Range[4000], !PrimeQ[ # ]&]]; Sqrt[Intersection[tp, tc]-1]
CROSSREFS
Sequence in context: A348394 A158713 A142185 * A046872 A167860 A152988
KEYWORD
more,nonn
AUTHOR
T. D. Noe, Mar 30 2009
STATUS
approved