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 A131981 Number of early bird numbers <= n. 4

%I

%S 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 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 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 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 (JBASIC)

%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 ; ",";

%o NEXT

%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).

%K nonn,base

%O 1,21

%A _Klaus Brockhaus_, Aug 15 2007

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Last modified May 25 02:01 EDT 2020. Contains 334581 sequences. (Running on oeis4.)