A378512 Numbers k such that 6^sigma(k) - k is a prime.
1, 7, 13, 77, 395, 2867, 3959, 5023
Offset: 1
Examples
7 is in the sequence because 6^sigma(7) - 7 = 6^8 - 7 = 1679609 is prime.
Crossrefs
Programs
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Magma
[n: n in[1..10000] | IsPrime((6^SumOfDivisors(n)) - n)];
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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] -
PARI
isok(k) = ispseudoprime(6^sigma(k) - k); \\ Michel Marcus, Dec 09 2024
Comments