A377786 Numbers k such that 5^sigma(k) - k is a prime.

4, 524, 7206, 11156

Offset: 1

Author

J.W.L. (Jan) Eerland, Nov 11 2024

Keywords

• nonn
• more

Examples

			4 is in the sequence because 5^sigma(4) - 4 = 5^7 - 4 = 78121 is prime.
		

Crossrefs

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

Programs

Magma

[n: n in[1..10000] | IsPrime((5^SumOfDivisors(n)) - n)];

Mathematica

a[n_] := Select[Range@ n, PrimeQ[5^DivisorSigma[1, #] - #] &]; a[20000]
DeleteCases[ParallelTable[If[PrimeQ[5^DivisorSigma[1,k]-k],k,n],{k,1,10^4}],n]

Extensions

a(4) from Michael S. Branicky, Nov 11 2024