The double factorial of a positive integer is defined by:
Find the number of ways the value of can be expressed as , where and are positive integers and .
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Let us look at the value of 2 0 1 2 ! ! 2 0 1 4 ! more closely. This simplifies to 2 0 1 3 ! ! ⋅ 2 0 1 4 . Notice that 2 1 is the largest power of 2 for this value, since 2 0 1 3 ! ! is a product of odd numbers and 2 0 1 4 = 2 ⋅ 1 9 ⋅ 5 3 . Furthermore, a 2 − b 2 = ( a + b ) ( a − b ) , and in order for a and b to be both integers, a + b and a − b must have the same parity (proof: the difference between a + b and a − b is 2 b , which is even). However, a + b and a − b cannot have the same parity, because only one can be even while the other is odd. Therefore, a and b cannot be positive integers, so there are 0 possible ways.