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 A265425 Numbers n such that n+2 and sigma(n-1) are both primes. 0
 3, 5, 17, 65, 4097, 65537, 262145, 1073741825 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS If a(9) exists, it must be larger than A023194(10000) = 5896704025969. Prime terms: 3, 5, 17, 65537, ... Any prime present must be one of the lesser twin primes (A001359) and also a Fermat prime (A019434), at least. See comments in A023194. - Antti Karttunen, Dec 08 2015 Sequence is different from A256438; numbers 1152921504606846977, 309485009821345068724781057, 81129638414606681695789005144065 and 85070591730234615865843651857942052865 are not terms of this sequence. Numbers 2^m+1 such that 2^m + 3 and 2^(m+1) - 1 are both prime. - Hiroaki Yamanouchi, Jan 04 2016 LINKS EXAMPLE Number 17 is in the sequence because 17 + 2 = 19 and sigma(17-1) = sigma(16) = 31; 17 and 31 are primes. MATHEMATICA Select[Range[10^7], And[PrimeQ[# + 2], PrimeQ[DivisorSigma[1, # - 1]]] &] (* Michael De Vlieger, Dec 09 2015 *) PROG (MAGMA) [n: n in [2..1000000] | IsPrime(n+2) and IsPrime(SumOfDivisors(n-1))] (PARI) for(n=2, 10^7, if(ispseudoprime(n+2) && ispseudoprime(sigma(n-1)), print1(n, ", "))) \\ Altug Alkan, Dec 08 2015 CROSSREFS Cf. A000203, A001359, A019434, A023194, A256438. Sequence in context: A085749 A281623 A278741 * A256438 A251737 A125957 Adjacent sequences:  A265422 A265423 A265424 * A265426 A265427 A265428 KEYWORD nonn,more AUTHOR Jaroslav Krizek, Dec 08 2015 STATUS approved

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Last modified June 25 03:58 EDT 2019. Contains 324338 sequences. (Running on oeis4.)