%N Number of early bird numbers <= n.
%C a(n) = number of k such that A116700(k) <= n; a(n) = n - number of k such that A131881(k) <= n.
%C 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 Klaus Brockhaus, <a href="/A131981/b131981.txt">Table of n, a(n) for n = 1..6000</a>
%H Klaus Brockhaus, <a href="/A131981/a131981.gif">Plots of A131981(n)/n at various scales</a>
%e There are two early bird numbers <= 21, viz. 12 and 21, hence a(21) = 2.
%o s$ = "" : d = 0
%o FOR n = 1 TO 84
%o sn$ = str$(n)
%o IF instr(s$, sn$) > 0 THEN d = d+1
%o s$ = s$ + sn$ : print d ; ",";
%Y 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).
%A _Klaus Brockhaus_, Aug 15 2007