A216093 a(n) = 10^n - (5^(2^n) mod 10^n).

5, 75, 375, 9375, 9375, 109375, 7109375, 87109375, 787109375, 1787109375, 81787109375, 81787109375, 81787109375, 40081787109375, 740081787109375, 3740081787109375, 43740081787109375, 743740081787109375

Offset: 1

Author

V. Raman, Sep 01 2012

Keywords

• nonn

Comments

a(n)^3 mod 10^n = a(n).

a(n) is the unique positive integer less than 10^n such that a(n) is divisible by 5^n and a(n) + 1 is divisible by 2^n. - Eric M. Schmidt, Sep 01 2012

a(n+1) + a(n)^2 == 0 (mod 10^(n+1)). - Robert Israel, Apr 24 2017

Links

• Robert Israel, Table of n, a(n) for n = 1..999

Crossrefs

Cf. A007185, A016090, A216092, A091663, A018248.

Programs

Maple

f:= n -> (-5 &^(2^n) mod 10^n):
map(f, [$1..30]); # Robert Israel, Apr 24 2017

Sage

def A216093(n) : return crt(-1, 0, 2^n, 5^n) # Eric M. Schmidt, Sep 01 2012

Formula

2^(4*5^(n-1)) mod 10^n - 1.