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A137233 Number of n-digit even numbers. 0
5, 45, 450, 4500, 45000, 450000, 4500000, 45000000, 450000000, 4500000000, 45000000000, 450000000000, 4500000000000, 45000000000000, 450000000000000, 4500000000000000, 45000000000000000, 450000000000000000 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

At Macau-Zhuhai immigration, there are total 24 rows: 1 23 45 67 89 10 11 1213 1415 1617 1819 2021 2223 24. a(n) is also the total number of double rows identified numbers in n digit.

For example:

n = 1: 01 23 45 67 89 = 5 double rows;

n = 2: 1011 1213 1415 1617 1819...9899 = 45 double rows;

n = 3: 100101 102103 104105...998999 = 450 double rows;

...

The double rows number is also A030656. [Kival Ngaokrajang, Oct 18 2013]

LINKS

Table of n, a(n) for n=1..18.

Wikipedia, Gongbei Port of Entry

FORMULA

a(n)=9*10^(n-1)/2 if n > 1. - R. J. Mathar, May 23 2008

EXAMPLE

a(2) = 45 because there are 45 2-digit even numbers.

CROSSREFS

Cf. A002275, A090843, A097166, A099914, A099915, A037487, A000422, A057138, A014925, A014923, A016313, A002276, A011577, A019518, A053052.

Sequence in context: A121272 A054318 A093140 * A001449 A209442 A199753

Adjacent sequences:  A137230 A137231 A137232 * A137234 A137235 A137236

KEYWORD

easy,nonn,base

AUTHOR

Ctibor O. Zizka, Mar 08 2008

EXTENSIONS

Corrected and extended by R. J. Mathar, May 23 2008

STATUS

approved

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Last modified June 25 22:27 EDT 2019. Contains 324364 sequences. (Running on oeis4.)