Definition:Extended Real Multiplication

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Let $\overline \R$ denote the extended real numbers.

Define extended real multiplication or multiplication on $\overline \R$, denoted $\cdot_{\overline \R}: \overline \R \times \overline \R \to \overline \R$, by:

$\forall x \in \overline \R: x \cdot_{\overline \R} 0 = 0 \cdot_{\overline \R} x = 0$
$\forall x, y \in \R: x \cdot_{\overline \R} y := x \cdot_\R y$ where $\cdot_\R$ denotes real multiplication
$\forall x \in \R_{>0}: x \cdot_{\overline \R} \left({+\infty}\right) = \left({+\infty}\right) \cdot_{\overline \R} x := +\infty$
$\forall x \in \R_{<0}: x \cdot_{\overline \R} \left({+\infty}\right) = \left({+\infty}\right) \cdot_{\overline \R} x := -\infty$
$\forall x \in \R_{>0}: x \cdot_{\overline \R} \left({-\infty}\right) = \left({-\infty}\right) \cdot_{\overline \R} x := -\infty$
$\forall x \in \R_{<0}: x \cdot_{\overline \R} \left({-\infty}\right) = \left({-\infty}\right) \cdot_{\overline \R} x := +\infty$
$\left({+\infty}\right) \cdot_{\overline \R} \left({+\infty}\right) := +\infty$
$\left({-\infty}\right) \cdot_{\overline \R} \left({-\infty}\right) := +\infty$
$\left({+\infty}\right) \cdot_{\overline \R} \left({-\infty}\right) := -\infty$
$\left({-\infty}\right) \cdot_{\overline \R} \left({+\infty}\right) := -\infty$

When no danger of confusion arises, $\cdot_{\overline \R}$ is usually replaced with the more familiar $\cdot$, or even suppressed.

From the definition of $\cdot_{\overline \R}$ on bona fide real numbers, the name extended real multiplication is appropriate: the real multiplication is indeed extended.

Also see