How to prove $d>0$ is a divisor of $n$ iff $d=p_1^{b_1}p_2^{b_2}...p_r^{b_r}$ with $0< b_i<a_i$ for each $i$? here $n = p_1^{a_1}p_2^{a_2}...p_r^{a_r}$ with the $p_i$ distinct primes and the $a_i$ positive integers.
I am unsure of how to start this problem any solutions or hints are welcome