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 A236689 #16 Dec 23 2024 14:53:43 %S A236689 0,1,2,3,4,5,6,10,7,11,8,12,9,20,13,14,15,16,17,18,30,21,22,23,24,25, %T A236689 26,27,29,200,31,32,33,34,35,36,38,100,40,41,42,43,44,45,47,101,50,51, %U A236689 52,53,54,56,102,60,61,62,63,65,103,80,70,71,72,74,104 %N A236689 The sum of two neighboring digits is a palindrome; a(n) is the smallest possible nonnegative integer not occurring earlier. %C A236689 The sum of two digits is a palindrome iff it is less than 10 or equal to 11. Therefore, numbers with substrings 19, 28, 37, 39, 46, ... (this is not A065207) can never occur, and this is not a permutation of the nonnegative integers. %H A236689 E. Angelini, <a href="https://web.archive.org/web/*/http://list.seqfan.eu/oldermail/seqfan/2014-January/012397.html">Palindromes -- with digits</a>, SeqFan List, Jan 27 2014. %o A236689 (PARI) a=u=0;(isp(s)=s<10||s==11);for(n=1,100,print1(a",");u+=1<<a;for(k=1,9e9,bittest(u,k)&&next; for(j=1,#d=digits(k), isp(if(j>1,d[j-1],a%10)+d[j])&&next;k=(k\10^(#d-j)+1)*10^(#d-j)-1;next(2));a=k;break)) %Y A236689 Cf. A228730. %K A236689 nonn,base %O A236689 0,3 %A A236689 _Eric Angelini_ and _M. F. Hasler_, Jan 29 2014