Find minimal integer number , that period of fraction is .
Give , as answer.
This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try
refreshing the page, (b) enabling javascript if it is disabled on your browser and,
finally, (c)
loading the
non-javascript version of this page
. We're sorry about the hassle.
It's well-known that if gcd ( 1 0 , N ) = 1 , the period of N 1 is the multiplicative order of 1 0 mod N , i.e. the smallest positive exponent k such that 1 0 k ≡ 1 mod N . I'm not sure if there is an obvious way without brute force to enumerate the N for which the multiplicative order of 1 0 mod N is 9 6 6 , but a simple Python loop solved the problem immediately. I get the first five solutions to be 2 0 2 1 , 2 3 0 3 , 5 9 6 9 , 5 9 7 7 , 6 0 6 3 . So the answer is 2 0 2 1 .
(If N is not coprime to 1 0 , there will be some digits before the repeating part, but the period will be the same as the period of N / g c d ( 1 0 , N ) , so we get the minimal N when that gcd is 1 . )