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Assuming it's a2-1 you mean.
If a2-1 is even, it can be expressed as a2-1 = 2 * t, where t is a natural number.
a2-1 = (a - 1)(a + 1) = 2 * t
You immediately see that at least one of the terms in parenthesis has to be even, and then, that actually both have to be even, because (a + 1) - (a - 1) = 2.
Therefore, you can express both of the terms as:
a - 1 = 2 * s
a + 1 = 2 *r,
which gives us:
2 * s * 2 * r = a2 - 1, simplify:
4 * r * s = a2 - 1.
Therefore, if a2-1 is even, a2-1 is also divisible by 4.
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