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 A167523 #7 Oct 03 2017 03:11:45
%S A167523 4,8,12,13,15,18,30,33,37,46,54,55,56,58,59,62,63,66,70,74,78,83,84,
%T A167523 87,92,104,115,116,119,124,136,146,148,150,152,154,159,163,176,183,
%U A167523 184,187,192,210,211,212,213,215,216,217,220,221,225,226,230,231,235,236,240
%N A167523 Positions of zero digits in A167503 (when all terms are concatenated).
%C A167523 Also, positive integers not occurring in A167503 (bearing in mind that the terms of A167503 are written in base 3, but here numbers are written in base 10).
%e A167523 Sequence A167503: 1,2,10,12,20,21,100,101
%e A167523 Digit's position: 1 2 34 56 78 9A BCD EFG (A=10,...)
%e A167523 Thus the digits '0' occur at positions 4,8,12,13,15,...: this sequence.
%o A167523 (PARI) base(n,b=3,s=1) = { my( a=[ n%b ]); while( 0<n\=b, a=concat( n%b, a )); if( s, eval( Strchr( vectorsmall( #a,i,48+a[i] ))), a) }
%o A167523 {a=b=[]; for(n=1,99, #b>=n && for(i=a[n-1]+1,#b,b[i] && (a=concat(a,i)) && break); #a<n && a=concat(a,#b+1); b=concat(b,base(a[n],3,0))); for(i=1,#b, b[i] || print1(i","))}
%Y A167523 Cf. A167500-A167503.
%K A167523 nonn,base
%O A167523 1,1
%A A167523 _M. F. Hasler_, Nov 05 2009
%E A167523 Edited by _Charles R Greathouse IV_, Aug 02 2010