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%I #32 Nov 12 2022 03:40:14
%S 5,45,450,4500,45000,450000,4500000,45000000,450000000,4500000000,
%T 45000000000,450000000000,4500000000000,45000000000000,
%U 450000000000000,4500000000000000,45000000000000000,450000000000000000
%N Number of n-digit even numbers.
%C From _Kival Ngaokrajang_, Oct 18 2013: (Start)
%C a(n) is also the total number of double rows identified numbers in n digit.
%C For example:
%C n = 1: 01 23 45 67 89 = 5 double rows;
%C n = 2: 1011 1213 1415 1617 1819...9899 = 45 double rows;
%C n = 3: 100101 102103 104105...998999 = 450 double rows;
%C ...
%C The number of double rows is also A030656. (End)
%C a(n) is also the number of n-digit integers with an even number of even digits (A356929); a(5) = 45000 is the answer to the question 2 of the Olympiade Mathématique Belge in 2004 (link). - _Bernard Schott_, Sep 06 2022
%C a(n) is also the number of n-digit integers with an odd number of odd digits (A054684). - _Bernard Schott_, Nov 07 2022
%H Olympiade Mathématique Belge, <a href="http://omb.sbpm.be/modules/finale/article.php?storyid=86">OMB 2004, Finale Maxi, Question 2</a>.
%H <a href="/index/O#Olympiads">Index to sequences related to Olympiads</a>.
%H <a href="/index/Rec#order_01">Index entries for linear recurrences with constant coefficients</a>, signature (10).
%F a(n) = 9*10^(n-1)/2 if n > 1. - _R. J. Mathar_, May 23 2008
%e a(2) = 45 because there are 45 2-digit even numbers.
%o (Python)
%o def A137233(n): return 9*10**(n-1)+1>>1 # _Chai Wah Wu_, Nov 11 2022
%Y Cf. A002275, A090843, A097166, A099914, A099915, A037487, A000422, A057138, A014925, A014923, A016313, A002276, A011577, A019518, A053052.
%Y Cf. A054684, A356929.
%K easy,nonn,base
%O 1,1
%A _Ctibor O. Zizka_, Mar 08 2008
%E Corrected and extended by _R. J. Mathar_, May 23 2008
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