(1+x₁+x₁²+...+x₁^p) (1+x₂+x₂²+...+x₂^q) (1+x₃+x₃²+...+x₃^r)(1+x₄+x₄²+...+x₄^r)
Clearly, the powers of different x_i.vary from 0 to p,q,r in each term and each term will have a combination of all the x_is
this expansion contains all divisors of the number N and hence is the sum
of all the divisors