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 A217836 #37 Oct 20 2012 18:56:56
%S A217836 4,6,10,15,26,51,95,187,371,737,1473,2942,5878,11755,23507,47013,
%T A217836 94021,188041,376069,752135,1504261,3008503,6017001,12034001,24068001,
%U A217836 48135995,96271987,192543973,385087943,770175883,1540351763,3080703523,6161407045,12322814089,24645628171
%N A217836 a(n) is largest semiprime < 2*a(n-1), with a(1) = 4.
%C A217836 This is to Bertrand primes A006992 as semiprimes A001358 are to primes A000040.
%H A217836 Robert G. Wilson v, <a href="/A217836/b217836.txt">Table of n, a(n) for n = 1...225</a>
%e A217836 a(2) = 6 because that is the largest semiprime < 2*a(1) = 8, where a(1) is the first semiprime.
%e A217836 a(3) = 10, the largest semiprime < 2*6 = 12.
%t A217836 PrevSemiPrime[n_, k_] := Block[{c = 0, sp = n - 1}, While[c < k, While[ PrimeOmega[sp] != 2, sp--]; sp--; c++]; sp + 1]; NestList[ PrevSemiPrime[ 2#, 1] &, 4, 34] (* _Robert G. Wilson v_, Oct 19 2012 *)
%Y A217836 Cf. A001358, A006992.
%K A217836 nonn,easy
%O A217836 1,1
%A A217836 _Jonathan Vos Post_, Oct 19 2012
%E A217836 Terms greater than a(15) from _Robert G. Wilson v_, Oct 19 2012