OFFSET
1,2
COMMENTS
LINKS
E. Angelini, Strings resurrection, SeqFan mailing list, Sep 08 2012
FORMULA
A216587(n)=-1 if and only if n is in this sequence.
PROG
(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))}
CROSSREFS
KEYWORD
nonn,base,easy
AUTHOR
M. F. Hasler, Sep 09 2012
STATUS
approved