Let a_1, a_2,\cdots ,a_n be positive integers and let a be an integer greater than 1 and divisible
by the product a1a2⋯an. Prove that an+1+a−1 is not divisible by the product
(a + a_1 - 1)(a + a_2 - 1) \cdots (a + a_n - 1). number theory unsolvednumber theory