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 A216589 #9 Dec 23 2024 14:53:43
%S A216589 0,11,12,13,14,15,16,17,18,19,20,30,40,50,60,70,80,90,100,110,111,112,
%T A216589 113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,
%U A216589 130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147,148
%N A216589 Numbers n which don't have a preimage for A216556, i.e., such that A216587(n)=-1.
%C A216589 Concretely, numbers having "20", "30", ..., "90" or "00" as a substring, or starting with "11", "12", ... "19", and the number 0.
%C A216589 Note that A216557 does not necessarily yield 0 for these numbers, e.g., 10 has no preimage for A216556, but 210 does have, and 10 -> 21 -> ... -> 98 -> 109.
%H A216589 E. Angelini, <a href="https://web.archive.org/web/*/http://list.seqfan.eu/oldermail/seqfan/2012-September/010124.html">Strings resurrection</a>, SeqFan mailing list, Sep 08 2012
%F A216589 A216587(n)=-1 if and only if n is in this sequence.
%o A216589 (PARI) is_A216589(n)={n || return(1); n=Vec(Str(n)); n[1]<"2" & 1<#n & n[2]>"0" & return(1); for(i=2,#n, n[i]=="0" || next; n[i-1]=="1" || return(1))}
%K A216589 nonn,base,easy
%O A216589 1,2
%A A216589 _M. F. Hasler_, Sep 09 2012