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 A210999 #13 Oct 08 2012 21:36:55 %S A210999 6,12,18,24,25,26,27,28,32,33,34,35,36,42,48,49,50,51,52,60,68,69,70, %T A210999 74,75,76,77,78,84,85,86,87,88,98,99,100,104,105,106,110,111,112,128, %U A210999 129,130,138,150,158,159,160,161,162,168,169,170,171,172 %N A210999 Composite numbers that are in the gap between an odd-indexed prime and an even-indexed prime. %H A210999 Harvey P. Dale, <a href="/A210999/b210999.txt">Table of n, a(n) for n = 1..1000</a> %e A210999 Illustration of initial terms of three sequences: %e A210999 A210998: 4, 8, 9, 10, 14, 15, 16 %e A210999 A000040: 2, 3, 5, 7, 11, 13, %e A210999 A210999: 6, 12, %t A210999 Select[Range[4, 200], Not[PrimeQ[#]] && OddQ[PrimePi[#]] &] (* _Alonso del Arte_, Sep 26 2012 *) %t A210999 Flatten[Range[First[#]+1,Last[#]-1]&/@Partition[Prime[Range[3,50]],2]] (* _Harvey P. Dale_, Oct 08 2012 *) %Y A210999 Cf. A002808, A014689, A031215, A031368, A073170, A210998. %K A210999 nonn,easy %O A210999 1,1 %A A210999 _Omar E. Pol_, Sep 26 2012