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 A171255 #11 Dec 23 2024 14:53:42 %S A171255 5,17,19,67,79,4099,4111,4159,65537,65539,65551,65599,262147,266239, %T A171255 1073741827,1073741839,1073807359,309485009821345068724785151 %N A171255 Primes which are the average of two distinct Mersenne primes (A000668). %C A171255 The subsequence of primes in A171253, which equals A171254 minus its subsequence A000668. %H A171255 Jonathan Vos Post, <a href="https://web.archive.org/web/*/http://list.seqfan.eu/oldermail/seqfan/2010-March/003808.html">Sums of three Mersenne primes, and prime sums of three Mersenne primes</a>, SeqFan list, Mar 5, 2010. %F A171255 A171255 = A171253 intersect A000040 = A171254 \ A000668. %e A171255 a(n) = A171253(n) for n=1,2,3, since all of these terms are prime. The term A171253(4) = 65 is the first element of A171252 to be composite, and therefore not included in the present sequence A171255. %o A171255 (PARI) select(isprime, concat(vector(#A00668,i,vector(i-1,j,A00668[i]+A00668[j])))/2) /* having defined A00668 as vector with initial terms of A000668. In PARI version 2.4.2, the syntax select( concat(...), x->isprime(x)) must be used. */ %Y A171255 Cf. A171252, A171254 (includes elements of A000668). %K A171255 nonn %O A171255 1,1 %A A171255 _M. F. Hasler_, Mar 06 2010