Gaussian Integral

Theorem
Let $\phi_2$ be the Gaussian Integral of Two Variables:


 * $\phi_2: \left\{ \left({a,b}\right)\in \R^2: a \le b \right \} \to \R$:


 * $\phi_2\left({a,b}\right) = \displaystyle \int_a^b \frac 1 {\sqrt{2\pi}} \exp \left({-\frac 1 2t^2 }\right) \, \mathrm dt$

Then the value of the improper integral of $\phi_2$ over the reals is one:


 * $\phi_2\left({-\infty,+\infty}\right) = \displaystyle \int_{\to -\infty}^{\to +\infty} \frac 1 {\sqrt{2\pi}} \exp \left({-\frac 1 2t^2 }\right) \, \mathrm dt = 1$

Equivalently:


 * $\displaystyle \lim_{x \to +\infty} \phi_1 \left({x}\right) = 1$

where $\phi_1$ is the Gaussian Integral of One Variable.