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 A321800 #13 Nov 20 2018 22:55:33
%S A321800 -1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,11,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,22,
%T A321800 -1,-1,-1,-1,-1,-1,-1,-1,-1,-1,33,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,44,-1,
%U A321800 -1,-1,-1,-1,-1,-1,-1,-1,-1,55,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,66,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,77,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,88,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,99,0,11,-1,-1,-1,-1,-1,-1,-1,-1,11,111,11,11,11,11,11,11,11,11,-1,11,22,-1,-1,-1,-1,-1,-1,-1,-1,11,-1,33,-1,-1,-1,-1,-1,-1,-1,11,-1,-1,44,-1,-1,-1,-1,-1,-1,11,-1,-1,-1,55,-1,-1,-1,-1,-1,11,-1,-1,-1,-1,66,-1,-1,-1,-1,11,-1,-1,-1,-1,-1,77,-1,-1,-1,11,-1,-1,-1,-1,-1,-1,88,-1,-1,11,-1,-1,-1,-1,-1,-1,-1,99
%N A321800 Delete the decimal digits of n that appear exactly once; write -1 if all digits disappear.
%C A321800 A companion sequence to A320485.
%C A321800 Digits that appear exactly once in n are erased. Leading zeros are erased unless the result is 0. If all digits are erased, we write -1 for the result (A321797 is another version, which uses 0 for the empty string).
%C A321800 More than the usual number of terms are shown in order to reach some interesting examples.
%H A321800 Chai Wah Wu, <a href="/A321800/b321800.txt">Table of n, a(n) for n = 0..10000</a>
%e A321800 12321 becomes 1221, 1123 becomes 11, 11231 becomes 111, and 100223 becomes 22 (as we don't accept leading zeros). Note that 12345 disappears immediately and we get -1.
%o A321800 (Python)
%o A321800 def A321800(n):
%o A321800 return (lambda x: int(x) if x != '' else -1)(''.join(d if str(n).count(d) != 1 else '' for d in str(n)))
%Y A321800 Cf. A320485, A321797.
%K A321800 sign,base,look
%O A321800 0,12
%A A321800 _Chai Wah Wu_, Nov 19 2018