# Dirac delta equality

Does the equality below hold?

$$f(x)=\int_{-\infty}^{\infty}f(x')\delta(x-x')dx=\int_{-\infty}^{\infty}f(x')\delta(x'-x)dx$$

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Why this is true? how could you get $f(x)$ be integrating with variable $dx$? –  Bombyx mori Nov 13 '12 at 0:44

Well, $$\int_{\mathbb R} f(x)\delta (x - a)dx = {f(a)\int_{- \infty }^\infty {\delta (x - a)dx}} = f(a) = {f(a)\int_{- \infty }^\infty {\delta (a - x)dx}}$$ so YES, if you change your $dx$ to $dx'$.

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for both sides $dx$ by $dx'$? –  user46060 Nov 13 '12 at 1:16
Yes. You should have: $f(x) = \int {f(x')\delta (x' - x)dx' = \int {f(x')\delta (x - x')dx'}}$. You could also change $a$ (in my answer) to $x'$. It is just notation. Whatever you prefer. –  glebovg Nov 13 '12 at 1:37

The correct form should be:

$f(y)=\int_{-\infty}^{\infty}f(x) \delta(x-y)dx = \int_{-\infty}^{\infty}f(x) \delta(y-x)dx$

When $x=y$, we have $\delta(y-y)=\delta(0)$ and this performs a sampling on the $f(x)$ function at the point $x=y$.

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Yes since the argument of the delta function equals zero when x=y in either case, and the result is f(x) for either integral.

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