

A151754


Number of ndigit numbers that are divisible by 5^n.


2



1, 3, 7, 14, 28, 57, 115, 230, 460, 921, 1843, 3686, 7372, 14745, 29491, 58982, 117964, 235929, 471859, 943718, 1887436, 3774873, 7549747, 15099494, 30198988, 60397977, 120795955, 241591910, 483183820, 966367641
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OFFSET

1,2


LINKS

Table of n, a(n) for n=1..30.
Index entries for linear recurrences with constant coefficients, signature (3,3,3,2).


FORMULA

Lim n > inf. a(n+1}/a{n) = 2.
a(n) = [(9/10)*2^n].  David W. Wilson, Jun 18 2009
G.f. ( 1+x^2x^3 ) / ( (x1)*(2*x1)*(x^2+1) ).  R. J. Mathar, Feb 20 2011


EXAMPLE

a(1)=1 because 5 divides only 5,
a(2)=3 because 25 divides {25, 50 & 75},
a(3)=7 because 125 divides {125, 250, 375, 500, 625, 750 & 925}, etc.


MATHEMATICA

f[n_] := Floor[(10^n  1)/5^n]  Floor[10^(n  1)/5^n]; Array[f, 35]
LinearRecurrence[{3, 3, 3, 2}, {1, 3, 7, 14}, 30] (* Harvey P. Dale, Feb 20 2016 *)


CROSSREFS

Cf. A151752.
Sequence in context: A153234 A293334 A266625 * A192065 A157672 A125899
Adjacent sequences: A151751 A151752 A151753 * A151755 A151756 A151757


KEYWORD

nonn,base


AUTHOR

Robert G. Wilson v, Jun 17 2009


STATUS

approved



