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Primes p such that there is a composite c with sigma_2(p)=sigma_2(c).
1

%I #2 Mar 30 2012 17:22:54

%S 7,47,157,3863

%N Primes p such that there is a composite c with sigma_2(p)=sigma_2(c).

%C 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.

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

%K more,nonn

%O 1,1

%A _T. D. Noe_, Mar 30 2009