Arithmetic

Every integer greater than 1 can be represented uniquely as a product of prime numbers, i.e.

$$n=p_1^{d_1} p_1^{d_2}...p_k^{d_k},$$

where

$$p_k \in prime, d_k \in \mathbb{N}$$

Algebra

Every non-constant single-variable polynomial with complex coefficients has at least one complex root. This includes polynomials with real coefficients since every real number is a complex number with its imaginary part equal to zero.