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A216099 Period of powers of 3 mod 10^n. 3
4, 20, 100, 500, 5000, 50000, 500000, 5000000, 50000000, 500000000, 5000000000, 50000000000, 500000000000, 5000000000000, 50000000000000, 500000000000000, 5000000000000000, 50000000000000000, 500000000000000000 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Table of n, a(n) for n=1..19.

FORMULA

4*5^(n-1), n <= 4.

5*10^(n-2), n >= 5.

EXAMPLE

a(3) = 100 because 3^100 = 515377520732011331036461129765621272702107522001 = 1 mod 1000.

MATHEMATICA

Table[If[n<5, (4*5^n)/5, 10^n/20], {n, 20}] (* or *) Join[{4, 20, 100}, NestList[ 10#&, 500, 20]] (* Harvey P. Dale, May 31 2017 *)

PROG

(PARI) a(n)=if(n<5, 4*5^n/5, 10^n/20) \\ Charles R Greathouse IV, Mar 26 2016

CROSSREFS

Cf. A000244, A001148, A001218.

Sequence in context: A178874 A103771 A005054 * A105480 A242156 A186369

Adjacent sequences:  A216096 A216097 A216098 * A216100 A216101 A216102

KEYWORD

nonn,base,easy

AUTHOR

V. Raman, Sep 01 2012

STATUS

approved

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Last modified February 22 09:34 EST 2019. Contains 320389 sequences. (Running on oeis4.)