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A270415
Numbers n such that sigma(n-1) and sigma(n) - 1 are both primes.
2
3, 5, 10, 17, 26, 65, 65537, 146690, 703922, 1481090, 1885130, 2036330, 2211170, 2430482, 2505890, 5470922, 9840770, 11607650, 17783090, 24137570, 74425130, 76615010, 77563250, 133379402, 138697730, 138980522, 142396490, 155575730, 177715562, 181899170
OFFSET
1,1
COMMENTS
Numbers n such that A000203(n-1) and A039653(n) are both primes.
Intersection of A270413 and A248792.
Prime terms are in A249759.
Corresponding values of sigma(n-1): 3, 7, 13, 31, 31, 127, 131071, ...
Corresponding values of sigma(n) - 1: 3, 5, 17, 17, 41, 83, 65537, ...
EXAMPLE
17 is in the sequence because sigma(17-1) = sigma(16) = 31 and sigma(10) - 1 = 18 - 1 = 17 (both primes).
MATHEMATICA
Select[Range[10^7], And[PrimeQ@ DivisorSigma[1, # - 1], PrimeQ[DivisorSigma[1, #] - 1]] &] (* Michael De Vlieger, Mar 17 2016 *)
PROG
(Magma) [n: n in [2..10000000] | IsPrime(SumOfDivisors(n-1)) and IsPrime(SumOfDivisors(n)-1)]
(PARI) isok(n) = isprime(sigma(n-1)) && isprime(sigma(n)-1); \\ Michel Marcus, Mar 17 2016
CROSSREFS
KEYWORD
nonn
AUTHOR
Jaroslav Krizek, Mar 16 2016
STATUS
approved