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 A037461 #18 Mar 01 2014 09:44:04
%S A037461 1,2,3,7,8,9,10,14,15,16,17,21,22,23,24,49,50,51,52,56,57,58,59,63,64,
%T A037461 65,66,70,71,72,73,98,99,100,101,105,106,107,108,112,113,114,115,119,
%U A037461 120,121,122,147,148,149,150,154,155,156,157
%N A037461 a(n)=Sum{d(i)*7^i: i=0,1,...,m}, where Sum{d(i)*4^i: i=0,1,...,m} is the base 4 representation of n.
%C A037461 A number k is a term of this sequence if and only if 7 divides neither C(2*k-1,k) nor C(2*k,k).
%H A037461 Clark Kimberling, <a href="/A037461/b037461.txt">Table of n, a(n) for n = 1..1000</a>
%e A037461 39 = 3*1 + 1*4 + 2*4^2 -> 3*1 + 1*7 + 3*7^2 = 108, so a(39) = 108. - _Clark Kimberling_, Jul 30 2012
%t A037461 Table[FromDigits[RealDigits[n,4],7],{n,1,100}]
%t A037461 (* _Clark Kimberling_, Aug 02 2012 *)
%Y A037461 Cf. A050608.
%K A037461 nonn,base
%O A037461 1,2
%A A037461 _Clark Kimberling_