Existence and Uniqueness of Positive Root of Positive Real Number
Theorem
Let $x \in \R$ be a real number such that $x \ge 0$.
Let $n \in \Z$ be an integer such that $n \ne 0$.
Then there always exists a unique $y \in \R: \paren {y \ge 0} \land \paren {y^n = x}$.
Hence the justification for the terminology the positive $n$th root of $x$ and the notation $x^{1/n}$.
Positive Exponent
Let $x \in \R$ be a real number such that $x \ge 0$.
Let $n \in \Z$ be an integer such that $n > 0$.
Then there always exists a unique $y \in \R: \paren {y \ge 0} \land \paren {y^n = x}$.
Negative Exponent
Let $x \in \R$ be a real number such that $x \ge 0$.
Let $n \in \Z$ be an integer such that $n < 0$.
Then there always exists a unique $y \in \R: \paren {y \ge 0} \land \paren {y^n = x}$.
Proof
The result follows from Existence of Positive Root of Positive Real Number and Uniqueness of Positive Root of Positive Real Number.
$\blacksquare$
Sources
- 1977: K.G. Binmore: Mathematical Analysis: A Straightforward Approach ... (previous) ... (next): $\S 12.11$