This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A265425 #34 Sep 08 2022 08:46:14 %S A265425 3,5,17,65,4097,65537,262145,1073741825 %N A265425 Numbers n such that n+2 and sigma(n-1) are both primes. %C A265425 If a(9) exists, it must be larger than A023194(10000) = 5896704025969. %C A265425 Prime terms: 3, 5, 17, 65537, ... %C A265425 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 %C A265425 Sequence is different from A256438; numbers 1152921504606846977, 309485009821345068724781057, 81129638414606681695789005144065 and 85070591730234615865843651857942052865 are not terms of this sequence. %C A265425 Numbers 2^m+1 such that 2^m + 3 and 2^(m+1) - 1 are both prime. - _Hiroaki Yamanouchi_, Jan 04 2016 %e A265425 Number 17 is in the sequence because 17 + 2 = 19 and sigma(17-1) = sigma(16) = 31; 17 and 31 are primes. %t A265425 Select[Range[10^7], And[PrimeQ[# + 2], PrimeQ[DivisorSigma[1, # - 1]]] &] (* _Michael De Vlieger_, Dec 09 2015 *) %o A265425 (Magma) [n: n in [2..1000000] | IsPrime(n+2) and IsPrime(SumOfDivisors(n-1))] %o A265425 (PARI) for(n=2, 10^7, if(ispseudoprime(n+2) && ispseudoprime(sigma(n-1)), print1(n, ", "))) \\ _Altug Alkan_, Dec 08 2015 %Y A265425 Cf. A000203, A001359, A019434, A023194, A256438. %K A265425 nonn,more %O A265425 1,1 %A A265425 _Jaroslav Krizek_, Dec 08 2015