A378512 Numbers k such that 6^sigma(k) - k is a prime.

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

Offset

1,2

Comments

a(9) > 10^5. - Michael S. Branicky, Dec 01 2024

Links

Table of n, a(n) for n=1..8.

Example

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

Mathematica

a[n_] := Select[Range@ n, PrimeQ[6^DivisorSigma[1, #] - #] &]; a[20000]
DeleteCases[ParallelTable[If[PrimeQ[6^DivisorSigma[1, k]-k], k, n], {k, 1, 10^4}], n]

Prog

(Magma) [n: n in[1..10000] | IsPrime((6^SumOfDivisors(n)) - n)];
(PARI) isok(k) = ispseudoprime(6^sigma(k) - k); \\ Michel Marcus, Dec 09 2024

Crossrefs

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

Sequence in context: A174878 A177198 A177163 * A268350 A061521 A362246

Adjacent sequences: A378509 A378510 A378511 * A378513 A378514 A378515

Keyword

nonn,more

Author

J.W.L. (Jan) Eerland, Nov 29 2024

Status

approved