Peculiar mod

If n n is a positive integer, denote f ( n ) f (n) to be the number of positive integers k k such that n n and k k leave the same remainder when divided by 2 k 1 2k-1 .

Find the number of positive integers x , 1 x 2014 x, 1 \leq x \leq 2014 such that f ( x ) f (x) is odd.


The answer is 32.

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.

2 solutions

Patrick Corn
Apr 8, 2015

( 2 k 1 ) ( n k ) ( 2 k 1 ) ( b 1 ) = n k ( 2 k 1 ) ( 2 b 1 ) = 2 n 1 (2k-1) | (n-k) \Leftrightarrow (2k-1)(b-1) = n-k \Leftrightarrow (2k-1)(2b-1) = 2n-1 for some b 1 b \ge 1 . So f ( n ) f(n) equals the number of positive divisors of 2 n 1 2n-1 . It's odd if and only if 2 n 1 2n-1 is a square. For n 2014 n \le 2014 it can be 1 2 , 3 2 , , 6 3 2 1^2, 3^2, \ldots, 63^2 . There are 32 \fbox{32} such n n .

Jayanta Mandi
Sep 12, 2014

n has to be of the form 2 ( x 2 x ) + 1 2(x^{2}-x)+1 for x= 1,2,3,....

for n ≤2014 x can be upto 32.

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...