|
%I #24 Aug 20 2024 16:01:24
%S 3,5,17,49,53,185,503,1301,1689,1797,5929,14747,20433,29903,42137,
%T 64763
%N Numbers k such that 2^sigma(k) - k is a prime.
%C If it exists, a(17) > 120000. - _Michael S. Branicky_, Aug 19 2024
%e 5 is in the sequence because 2^sigma(5)-5 = 2^6-5 = 59 is prime.
%t a[n_] := Select[Range@ n, PrimeQ[2^DivisorSigma[1, #] - #] &]; a[20000]
%t DeleteCases[ParallelTable[If[PrimeQ[2^DivisorSigma[1,k]-k],k,n],{k,1,10^4}],n]
%o (Magma) [n: n in[1..10000] | IsPrime((2^SumOfDivisors(n)) - n)];
%Y Cf. A000043, A000203, A000668, A023194, A023195, A253850, A253851.
%K nonn,more,changed
%O 1,1
%A _J.W.L. (Jan) Eerland_, Jan 02 2024
%E a(16) from _J.W.L. (Jan) Eerland_, Jan 25 2024
|
