Let \(n\) be a natural number greater than \(1\). Suppose \( \left ( \mathbb{Z}_n, +, \cdot \right ) \) be a ring of integers mod \(n\). Then, every element \(a \in \mathbb{Z}_n \) has multiplicative inverse if and only if \(n\) is a prime number. In other words, \( \left ( \mathbb{Z}_n, +, \cdot \right ) \) is a field if and only if \(n\) is a prime number.