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%I #25 Sep 08 2022 08:46:10
%S 2,3,5,11,7,17,11,13,23,23,17,19,41,31,23,59,41,29,71,31,47,53,47,37,
%T 59,89,41,43,83,71,47,71,97,53,71,79,89,59,167,61,103,83,67,71,73,113,
%U 139,167,79,83,223,107,131,179,89,233,167,127,251,97,101,103
%N Sigma(n) - 1 for n such that sigma(n) - 1 is prime.
%C a(n) = corresponding values of primes p = sigma(A248792(n)) - 1, where A248792(n) = numbers n such that sigma(n) - 1 is prime.
%C If there are at least two numbers k, h such that a(k) = a(h) = p, then p is in A158913.
%H Michael De Vlieger, <a href="/A248793/b248793.txt">Table of n, a(n) for n = 1..10000</a>
%H OEIS Wiki, <a href="https://oeis.org/wiki/Cyclotomic Polynomials at x=n, n! and sigma(n)">Cyclotomic Polynomials at x=n, n! and sigma(n)</a>
%F a(n) = A000203(A248792(n)) - 1.
%F If A248792(n) is a prime p, then a(n) = A248792(n) = p.
%p F:= proc(n)
%p local r;
%p r:= numtheory:-sigma(n)-1;
%p if isprime(r) then r else NULL fi
%p end proc:
%p seq(F(n),n=1..1000); # _Robert Israel_, Nov 02 2014
%t a248793[n_Integer] :=
%t Cases[DivisorSigma[1, #] - 1 & /@ Range[n], _?PrimeQ]; a248793[104] (* _Michael De Vlieger_, Nov 07 2014 *)
%o (Magma) [a: n in [1..1000] | IsPrime(a) where a is SumOfDivisors(n)-1]
%o (PARI) for(n=1,10^3,if(isprime(sigma(n)-1),print1(sigma(n)-1,", "))) \\ _Derek Orr_, Nov 01 2014
%Y Cf. A000203, A000040, A066073, A248792.
%K nonn,easy
%O 1,1
%A _Jaroslav Krizek_, Nov 01 2014
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