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%I #17 Sep 08 2022 08:46:10
%S 3,5519,116927,227663,263759,297023,488639,616079,1108127,2973239,
%T 10738223,24934079,25803839,73277879,95133239,117864119,264054383,
%U 265178591,285400559,443052479,634090679,644512703,644615399,688686959,717336839
%N Primes p such that p - 1 and (p + 1)/2 have the same sum of divisors.
%e 3 is in this sequence because sigma(3 - 1) = sigma((3 + 1)/2) and 3 is prime.
%t Select[Prime[Range[10^4]], DivisorSigma[1, # - 1] == DivisorSigma[1, (# + 1)/2] &] (* _Alonso del Arte_, Oct 31 2014 *)
%o (Magma) [p: p in PrimesInInterval(3, 100000000) | SumOfDivisors(p-1) eq SumOfDivisors((p+1) div 2)];
%o (PARI) lista(nn) = {forprime(p=3, nn, if (sigma(p-1)==sigma((p+1)/2), print1(p, ", ")););} \\ _Michel Marcus_, Oct 31 2014
%Y Cf. A000203, A171720, A249625.
%K nonn
%O 1,1
%A _Juri-Stepan Gerasimov_, Oct 31 2014