Definition:Power (Algebra)/Real Number/Definition 3/Binary Expansion
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Definition
Let $x \in \R_{>0}$ be a (strictly) positive real number.
Let $r \in \R$ be a real number.
First let $x > 1$.
Let $r$ be expressed in binary notation:
- $r = n \cdotp d_1 d_2 d_3 \ldots$
where $d_1, d_2, d_3 \ldots$ are in $\set {0, 1}$.
For $k \in \Z_{> 0}$, let $\psi_1, \psi_2 \in \Q$ be rational numbers defined as:
\(\ds \psi_1\) | \(=\) | \(\ds n + \sum_{j \mathop = 1}^k \frac {d_1} {2^k} = n + \frac {d_1} 2 + \cdots + \frac {d_k} {2^k}\) | ||||||||||||
\(\ds \psi_2\) | \(=\) | \(\ds \psi_1 + \dfrac 1 {2^k}\) |
Then $x^r$ is defined as the (strictly) positive real number $\xi$ defined as:
- $\ds \lim_{k \mathop \to \infty} x^{\psi_1} \le \xi \le x^{\psi_2}$
In this context, $x^{\psi_1}, x^{\psi_2}$ denote $x$ to the rational powers $\psi_1$ and $\psi_2$.
Next let $x < 1$.
Then $x^r$ is defined as:
- $x^r := \paren {\dfrac 1 x}^{-r}$
Finally, when $x = 1$:
- $x^r = 1$
Sources
- 1997: Donald E. Knuth: The Art of Computer Programming: Volume 1: Fundamental Algorithms (3rd ed.) ... (previous) ... (next): $\S 1.2.2$: Numbers, Powers, and Logarithms: Exercise $5$