# The divisor function and number of divisors.

Prove $\sigma$(n) $\equiv$ $d(n)$ mod 2 where $\sigma(n)$ is the divisor function and $d(n)$ is the number of divisors of n. So, this is equivalent to saying that $\frac{\sigma(n)}{d(n)}$ $\equiv$ 1 mod 2. So I just have to show that the fraction of $\sigma(n)$ and $d(n)$ is odd. We know that $\sigma(n) = \prod\sigma(p^{a_{i}}_{i})$ for each $p^{a_{i}}_{i}$ in the prime factorization on n. I also know that $d(n) = \prod(a_i + 1)$ for each $a_i$ in the prime factorization of n. So this will yield $\prod\frac{\sigma(p^{a_{i}}_{i})}{a_i + 1}$. I'm not quite sure how I'm able to simplify this further or manipulate it.

If I understand your proposed theorem correctly, it is not true. I am assuming you mean the sum of divisors function by $σ(n)$. As a counterexample, $σ(2)=3$ and $d(2)=2$. However, your theorem is true for $n$ with an even number of 2 factors, or, in other words, for odd $n$ or $n$ divisible by 4. In fact, your proposed theorem is false in all other cases ($n$ divisible by $2$ but not by $4$).
Let $n$ = $2^mp_1^{e_1}*\cdots*p_k^{e_k}$. Then $$σ(n)=(1+2+\cdots+2^m)(1+p_1+\cdots+p_1^{e_1})*\cdots*(1+p_k+\cdots+p_k^{e_k})$$ This has $(e_1+1)*\cdots*(e_k+1)$ odd terms, which is precisely the number of odd divisors of $n$. If $m$ is even, then the total number of divisors of $n$ is an odd multiple of the number of odd divisors. Thus, for odd $m$, $σ(n)$ is odd iff $d(n)$ is odd.