The inverse number or reciprocal number of a non-zero real or complex number $a$ may be denoted by $a^{-1}$ and it means the quotient $\frac{1}{a}$ (so, it is really the $-1^\mathrm{th}$ power of $a$ .

• Two numbers are inverse numbers of each other if and only if their product is equal to 1 (cf. opposite inverses).
• If $a$ ($\neq 0$ is given in a quotient form $\frac{b}{c}$ then its inverse number is simply $$\left(\frac{b}{c}\right)^{-1} = \frac{c}{b}.$$
• Forming the inverse number is also a multiplicative function, i.e. $$(bc)^{-1} = b^{-1}c^{-1}$$ (to be more precise, it is an automorphism of the multiplicative group of $\mathbb{R}$ resp. $\mathbb{C}$ .