If is a positive integer, denote to be the number of positive integers such that and leave the same remainder when divided by .
Find the number of positive integers such that is odd.
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( 2 k − 1 ) ∣ ( n − k ) ⇔ ( 2 k − 1 ) ( b − 1 ) = n − k ⇔ ( 2 k − 1 ) ( 2 b − 1 ) = 2 n − 1 for some b ≥ 1 . So f ( n ) equals the number of positive divisors of 2 n − 1 . It's odd if and only if 2 n − 1 is a square. For n ≤ 2 0 1 4 it can be 1 2 , 3 2 , … , 6 3 2 . There are 3 2 such n .