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 A171253 #9 Dec 23 2024 14:53:42 %S A171253 5,17,19,65,67,79,4097,4099,4111,4159,65537,65539,65551,65599,69631, %T A171253 262145,262147,262159,262207,266239,327679,1073741825,1073741827, %U A171253 1073741839,1073741887,1073745919,1073807359,1074003967 %N A171253 Semi-sums (average) of any two distinct Mersenne primes (A000668). %C A171253 Subsequence of A171252, excluding the terms of A000668 itself. %H A171253 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 A171253 a(n) = A171252(A014132(n)). %F A171253 A171253 = A171252 \ A000688. %e A171253 a(1) = (A000668(2)+A000668(1))/2, a(2) = (A000668(3)+A000668(1))/2, a(3) = (A000668(3)+A000668(2))/2, a(4) = (A000668(4)+A000668(1))/2, ,... %o A171253 (PARI) concat(vector(#A000668,i,vector(i-1,j,A000668[i]+A000668[j])))/2 /* having defined A000668 as vector with initial terms of A000668 */ %Y A171253 Cf. A171252 (includes terms of A000668), A171255 (primes in this sequence). %K A171253 nonn %O A171253 1,1 %A A171253 _M. F. Hasler_, Mar 06 2010