A367460
Numbers k such that 3^sigma(k) - k is a prime.
Original entry on oeis.org
1, 10, 52, 400, 2480, 7202, 28222
Offset: 1
10 is in the sequence because 3^sigma(10) - 10 = 3^18 - 10 = 387420479 is prime.
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[n: n in[1..10000] | IsPrime((3^SumOfDivisors(n)) - n)]
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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]
A377927
Numbers k such that 4^sigma(k) - k is a prime.
Original entry on oeis.org
1, 5, 17, 57, 675, 1329, 1425, 3803, 39617
Offset: 1
17 is in the sequence because 4^sigma(17) - 17 = 4^18 - 17 = 68719476719 is prime.
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[n: n in[1..10000] | IsPrime((4^SumOfDivisors(n)) - n)];
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a[n_] := Select[Range@ n, PrimeQ[4^DivisorSigma[1, #] - #] &]; a[20000]
DeleteCases[ParallelTable[If[PrimeQ[4^DivisorSigma[1,k]-k],k,n],{k,1,10^4}],n]
A377786
Numbers k such that 5^sigma(k) - k is a prime.
Original entry on oeis.org
4, 524, 7206, 11156
Offset: 1
4 is in the sequence because 5^sigma(4) - 4 = 5^7 - 4 = 78121 is prime.
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[n: n in[1..10000] | IsPrime((5^SumOfDivisors(n)) - n)];
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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]
A378512
Numbers k such that 6^sigma(k) - k is a prime.
Original entry on oeis.org
1, 7, 13, 77, 395, 2867, 3959, 5023
Offset: 1
7 is in the sequence because 6^sigma(7) - 7 = 6^8 - 7 = 1679609 is prime.
Cf.
A000043,
A000203,
A000668,
A023194,
A023195,
A253850,
A253851,
A368651,
A367460,
A377927,
A377786.
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[n: n in[1..10000] | IsPrime((6^SumOfDivisors(n)) - n)];
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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]
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isok(k) = ispseudoprime(6^sigma(k) - k); \\ Michel Marcus, Dec 09 2024
Showing 1-4 of 4 results.
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