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 A131881 #12 Oct 25 2019 12:39:18 %S A131881 1,2,3,4,5,6,7,8,9,10,11,13,14,15,16,17,18,19,20,22,24,25,26,27,28,29, %T A131881 30,33,35,36,37,38,39,40,44,46,47,48,49,50,55,57,58,59,60,66,68,69,70, %U A131881 77,79,80,88,90,100,102,103,104,105,106,107,108,109,113,114 %N A131881 Complement of A116700. Might be called "punctual birds". %C A131881 Numbers n that do not occur in the concatenation of 1,2,3...,n-1. %C A131881 Every power of 10 is a member, which proves that the sequence is infinite. - _N. J. A. Sloane_, Jul 23 2007 %C A131881 The asymptotic density of the sequence is zero. The number of k-digit terms is A132133 = (9, 45, 270, 2104, ...), k = 1, 2, .... These are the first difference of the indices of powers of 10, T = (1, 10, 55, 325, 2429, ...), which we get as partial sums if we prefix A132133(0) = 1 corresponding to the number 0. - _M. F. Hasler_, Oct 24 2019 %H A131881 M. F. Hasler, <a href="/A131881/b131881.txt">Table of n, a(n) for n = 1..2428</a> %e A131881 The first number not in this sequence is the early bird "12" which occurs as concatenation of 1 and 2. %o A131881 (PHP) $s="0"; for(; ++$i < 2000; $s .= $i) if( !strpos($s,"$i")) echo $i,", "; %Y A131881 Cf. A116700 (early birds), A132133 (number of n-digit terms). %Y A131881 Cf. A007376 (Barbier word ...,8,9,1,0,1,1,...), A033307 (Champernowne constant). %K A131881 nonn,base,easy %O A131881 1,2 %A A131881 _M. F. Hasler_, Jul 23 2007