Differences in Squares (with factorials)

The double factorial of a positive integer is defined by:

n ! ! = { 1 3 5 ( n 2 ) n , if n 1 ( m o d 2 ) 2 4 6 ( n 2 ) n , if n 0 ( m o d 2 ) n!! = \begin{cases} 1 \cdot 3 \cdot 5 \cdots (n-2) \cdot n, & \text{if }n \equiv 1 \pmod{2} \\ 2 \cdot 4 \cdot 6 \cdots (n-2) \cdot n, & \text{if }n \equiv 0 \pmod{2} \end{cases}

Find the number of ways the value of 2014 ! 2012 ! ! \dfrac{2014!}{2012!!} can be expressed as a 2 b 2 a^2-b^2 , where a a and b b are positive integers and a > b a>b .


The answer is 0.

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1 solution

Hahn Lheem
May 30, 2014

Let us look at the value of 2014 ! 2012 ! ! \dfrac{2014!}{2012!!} more closely. This simplifies to 2013 ! ! 2014 2013!! \cdot 2014 . Notice that 2 1 2^1 is the largest power of 2 2 for this value, since 2013 ! ! 2013!! is a product of odd numbers and 2014 = 2 19 53 2014=2 \cdot 19 \cdot 53 . Furthermore, a 2 b 2 = ( a + b ) ( a b ) a^2-b^2=(a+b)(a-b) , and in order for a a and b b to be both integers, a + b a+b and a b a-b must have the same parity (proof: the difference between a + b a+b and a b a-b is 2 b 2b , which is even). However, a + b a+b and a b a-b cannot have the same parity, because only one can be even while the other is odd. Therefore, a a and b b cannot be positive integers, so there are 0 \boxed{0} possible ways.

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