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 A131981 #6 Mar 30 2012 17:27:54 %S A131981 0,0,0,0,0,0,0,0,0,0,0,1,1,1,1,1,1,1,1,1,2,2,3,3,3,3,3,3,3,3,4,5,5,6, %T A131981 6,6,6,6,6,6,7,8,9,9,10,10,10,10,10,10,11,12,13,14,14,15,15,15,15,15, %U A131981 16,17,18,19,20,20,21,21,21,21,22,23,24,25,26,27,27,28,28,28,29,30,31,32 %N A131981 Number of early bird numbers <= n. %C A131981 a(n) = number of k such that A116700(k) <= n; a(n) = n - number of k such that A131881(k) <= n. %C A131981 A131982 gives numbers n such that a(n) = n/2, or numbers n such that (number of k such that A116700(k) <= n) = (number of k such that A131881(k) <= n). %H A131981 Klaus Brockhaus, <a href="/A131981/b131981.txt">Table of n, a(n) for n = 1..6000</a> %H A131981 Klaus Brockhaus, <a href="/A131981/a131981.gif">Plots of A131981(n)/n at various scales</a> %e A131981 There are two early bird numbers <= 21, viz. 12 and 21, hence a(21) = 2. %o A131981 (JBASIC) %o A131981 s$ = "" : d = 0 %o A131981 FOR n = 1 TO 84 %o A131981 sn$ = str$(n) %o A131981 IF instr(s$, sn$) > 0 THEN d = d+1 %o A131981 s$ = s$ + sn$ : print d ; ","; %o A131981 NEXT %Y A131981 Cf. A116700 (early bird numbers), A131881 (complement of A116700), A132133 (number of n-digit terms of 131881), A105390 (number of Rollman numbers <= n), A131982 (numbers n such that A131981(n) = n/2). %K A131981 nonn,base %O A131981 1,21 %A A131981 _Klaus Brockhaus_, Aug 15 2007