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A270413
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Numbers n such that sigma(n-1) is a prime.
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4
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3, 5, 10, 17, 26, 65, 290, 730, 1682, 2402, 3482, 4097, 5042, 7922, 10202, 15626, 17162, 27890, 28562, 29930, 65537, 83522, 85850, 146690, 262145, 279842, 458330, 491402, 531442, 552050, 579122, 597530, 683930, 703922, 707282, 734450, 829922, 1190282, 1203410
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OFFSET
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1,1
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COMMENTS
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Corresponding values of sigma(n-1): 3, 7, 13, 31, 31, 127, 307, 1093, ...
Conjecture: supersequence of A256438.
Conjecture: 31 is the only prime p such that p = sigma(x-1) = sigma(y-1) for distinct numbers x and y; 31 = sigma(17-1) = sigma(26-1).
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LINKS
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FORMULA
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EXAMPLE
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17 is in the sequence because sigma(17-1) = sigma(16) = 31 (prime).
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MATHEMATICA
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Select[Range[10^6], PrimeQ@ DivisorSigma[1, # - 1] &] (* Michael De Vlieger, Mar 17 2016 *)
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PROG
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(Magma) [n: n in [2..2000000] | IsPrime(SumOfDivisors(n-1))]
(PARI) isok(n) = isprime(sigma(n-1)); \\ Michel Marcus, Mar 17 2016
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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