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 A212909 #9 Jul 13 2013 12:04:27 %S A212909 17,26,29,31,33,35,38,42,44,49,50,51,52,57,58,60,62,64,65,68,70,72,73, %T A212909 77,78,79,81,84,85,88,89,91,95,99,100,102,103,105,106,107,110,112,114, %U A212909 116,119,120,121,124,125,126,128,129,130,132,135,142,143,144,146,147,150,154,156,160,162,170,175,177,178,180,182,184,192,195,196,197,202,204,205 %N A212909 Numbers n such that A212813(n) = 4. %D A212909 Bellamy, O. S.; Cadogan, C. C. Subsets of positive integers: their cardinality and maximality properties. Proceedings of the Tenth Southeastern Conference on Combinatorics, Graph Theory and Computing (Florida Atlantic Univ., Boca Raton, Fla., 1979), pp. 167--178, Congress. Numer., XXIII-XXIV, Utilitas Math., Winnipeg, Man., 1979. MR0561043 (82b:10006) %H A212909 Reinhard Zumkeller, <a href="/A212909/b212909.txt">Table of n, a(n) for n = 1..10000</a> %o A212909 (Haskell) %o A212909 import Data.List (elemIndices) %o A212909 a212909 n = a212909_list !! (n-1) %o A212909 a212909_list = map (+ 1) $ elemIndices 4 a212813_list %o A212909 -- _Reinhard Zumkeller_, May 30 2012 %Y A212909 Cf. A212813, A212814, A212815, A212816, A212908. %K A212909 nonn,fini %O A212909 1,1 %A A212909 _N. J. A. Sloane_, May 30 2012