A377927 - OEIS

%I #11 Nov 14 2024 08:23:32

%S 1,5,17,57,675,1329,1425,3803,39617

%N Numbers k such that 4^sigma(k) - k is a prime.

%e 17 is in the sequence because 4^sigma(17) - 17 = 4^18 - 17 = 68719476719 is prime.

%t a[n_] := Select[Range@ n, PrimeQ[4^DivisorSigma[1, #] - #] &]; a[20000]

%t DeleteCases[ParallelTable[If[PrimeQ[4^DivisorSigma[1,k]-k],k,n],{k,1,10^4}],n]

%o (Magma) [n: n in[1..10000] | IsPrime((4^SumOfDivisors(n)) - n)];

%Y Cf. A000043, A000203, A000668, A023194, A023195, A253850, A253851, A368651, A367460.

%K nonn,more

%O 1,2

%A _J.W.L. (Jan) Eerland_, Nov 11 2024

%E a(9) from _Michael S. Branicky_, Nov 11 2024