Complex Subtraction is Closed

Theorem
The set of complex numbers is closed under subtraction:
 * $\forall a, b \in \C: a - b \in \C$

Proof
From the definition of complex subtraction:
 * $a - b := a + \paren {-b}$

where $-b$ is the inverse for complex number addition.

From Complex Numbers under Addition form Abelian Group, it follows that:
 * $\forall a, b \in \C: a + \paren {-b} \in \C$

Therefore complex number subtraction is closed.