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 A212556 #6 May 21 2012 17:02:34
%S A212556 0,1,2,3,100,101,102,103,110,111,112,113,120,121,122,123,130,131,132,
%T A212556 133,200,201,202,203,210,211,212,213,220,221,222,223,230,231,232,233,
%U A212556 300,301,302,303,310,311,312,313,320,321,322,323,330,331,332,333,10000
%N A212556 Representations of nonnegative integers in base -4: The range of A007608.
%C A212556 Also: Numbers with an odd number of digits, using only digits 0 through 3.
%C A212556 (These numbers (or strings of digits) represent nonnegative integers in base -4, while the same type of numbers with an even number of digits represent negative numbers. See references in A007608 for more information.)
%o A212556 (PARI) {forstep(d=1,3,2, my(u=vector(d,i,10^(d-i))~); forvec(v=vector(d,i,[i==1 & d>1,3]),print1(v*u",")))}
%o A212556 (PARI) is_A212556(n)==(#n=Str(n))%2 & !setminus(Set(Vec(n)),Vec("0123"))
%Y A212556 The same as A007608 sorted in increasing order.
%K A212556 nonn,base
%O A212556 1,3
%A A212556 _M. F. Hasler_, May 20 2012