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A158913 Primes p such that there is a composite c with sigma(p) = sigma(c). 8


%S 11,17,23,31,41,47,53,59,71,79,83,89,97,103,107,113,127,131,139,151,

%T 167,179,181,191,223,227,233,239,251,263,269,271,293,307,311,359,383,

%U 389,419,431,433,439,443,449,467,479,491,503,521,557,569,571,587,593,599

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

%C See A158914 for the sequence for sigma_2.

%H Donovan Johnson, <a href="/A158913/b158913.txt">Table of n, a(n) for n = 1..10000</a>

%p with(numtheory): P:=proc(q) local k,n;

%p for n from 1 to q do if isprime(n) then for k from 1 to q do

%p if sigma(n)=sigma(k) then break; fi; od; if k<n then print(n);

%p fi; fi; od; end: P(10^9); # _Paolo P. Lava_, Aug 07 2015

%t tp=DivisorSigma[1,Select[Range[1000],PrimeQ]]; tc=DivisorSigma[1,Select[Range[1000],!PrimeQ[ # ]&]]; Intersection[tp,tc]-1

%o (Sage) [sigma(n)-1 for n in (2..600) if is_prime(sigma(n)-1) and n<sigma(n)-1<600] # _Giuseppe Coppoletta_, Dec 22 2014

%K nonn

%O 1,1

%A _T. D. Noe_, Mar 30 2009

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Last modified May 29 02:45 EDT 2022. Contains 354122 sequences. (Running on oeis4.)