The integral of mollifier function is $1 $

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\begin{eqnarray} u(x) = \begin{cases} e^{-\frac{1}{1-\|x\|^2}}& \text{ if } \|x\| < 1\\ 0& \text{ if } \|x\|\geq 1 \end{cases} \end{eqnarray} I am trying to show the integral of the mollifier function $u$ equals $1$. Help please. Thank you.