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 A262016 #21 Sep 08 2022 08:46:13
%S A262016 0,0,2,7,15,83,91,5163,86315,401323,14725035,65973803,2562459691,
%T A262016 37695987243,45627576875,9183900142123,39579412285995,
%U A262016 2598666482813483,28017829467228715,223446104928848427,275288604322335275,116723151945719253547,12606804937123633465899
%N A262016 a(n) = ( ( Sum_{k=1..n} 2^(prime(k)-1) ) mod n! ) / 2.
%C A262016 This sequence appears to be dense in semiprimes.
%H A262016 R. J. Cano, <a href="/A262016/b262016.txt">Table of n, a(n) for n = 1..100</a>
%t A262016 Table[Mod[Sum[2^(Prime[k] - 1), {k, n}], n!]/2, {n, 23}] (* _Michael De Vlieger_, Sep 09 2015 *)
%o A262016 (PARI) a(n)=sum(k=1,n,2^(prime(k)-1))%n!/2
%o A262016 for(i=1,30,print1(a(i), ", ")); \\ _R. J. Cano_, Sep 08 2015
%o A262016 (Magma) [(&+[2^(NthPrime(k)-1): k in [1..n]] mod Factorial(n))/2: n in [1..25]]; // _Bruno Berselli_, Sep 24 2015
%K A262016 nonn
%O A262016 1,3
%A A262016 _R. J. Cano_ and _M. F. Hasler_, Sep 08 2015