A378512 - OEIS

%I #8 Dec 21 2024 00:49:04

%S 1,7,13,77,395,2867,3959,5023

%N Numbers k such that 6^sigma(k) - k is a prime.

%C a(9) > 10^5. - _Michael S. Branicky_, Dec 01 2024

%e 7 is in the sequence because 6^sigma(7) - 7 = 6^8 - 7 = 1679609 is prime.

%t a[n_] := Select[Range@ n, PrimeQ[6^DivisorSigma[1, #] - #] &]; a[20000]

%t DeleteCases[ParallelTable[If[PrimeQ[6^DivisorSigma[1,k]-k],k,n],{k,1,10^4}],n]

%o (Magma) [n: n in[1..10000] | IsPrime((6^SumOfDivisors(n)) - n)];

%o (PARI) isok(k) = ispseudoprime(6^sigma(k) - k); \\ _Michel Marcus_, Dec 09 2024

%Y Cf. A000043, A000203, A000668, A023194, A023195, A253850, A253851, A368651, A367460, A377927, A377786.

%K nonn,more

%O 1,2

%A _J.W.L. (Jan) Eerland_, Nov 29 2024