A367460 Numbers k such that 3^sigma(k) - k is a prime.

1, 10, 52, 400, 2480, 7202, 28222

Offset: 1

Author

J.W.L. (Jan) Eerland, Jan 26 2024

Keywords

• nonn
• more

Comments

a(8) > 67569.

Examples

			10 is in the sequence because 3^sigma(10) - 10 = 3^18 - 10 = 387420479 is prime.
		

Crossrefs

Cf. A000043, A000203, A000668, A023194, A023195, A253850, A253851, A368651.

Programs

Magma

[n: n in[1..10000] | IsPrime((3^SumOfDivisors(n)) - n)]

Mathematica

a[n_] := Select[Range@ n, PrimeQ[3^DivisorSigma[1, #] - #] &]; a[20000]
DeleteCases[ParallelTable[If[PrimeQ[3^DivisorSigma[1,k]-k],k,n],{k,1,10^4}],n]

Extensions

a(7) from Michael S. Branicky, Jan 27 2024