# Filtering property of Dirac $\delta$ function

Evaluate $$\int_0^\infty f(x)\delta(x-1)dx$$where$$f(x)=\begin{cases}x^2,&0\le x<1\\\sin 2,&x=1\\x,&x>1\end{cases}$$

Attempt

Since the function is discontinuous at $$1$$, I couldn't directly say the answer would be $$f(1)$$. I considered the $$\delta$$-sequence$$\delta_k(x-1)=\begin{cases}\frac k2,& |x-1|<1/k\\0,&\text{otherwise}\end{cases}\\\int_0^\infty f(x)\delta_k(x-1)dx=\frac k2\Big[\int_{1-1/k}^1x^2dx+\int^{1+1/k}_1xdx\Big]\\=1+\frac12\Big(\frac1{3k^2}-\frac1{2k}\Big)$$

Then taking the limit as $$k\to\infty$$, I got the answer as $$1$$, which seems correct intuitively as it is the limit of the function at $$x=1$$. But the answer key gives the answer $$\sin 2=f(1)$$.

Is the key wrong or am I missing something?

Edit

The book defines the Dirac $$\delta$$ function (also called impulse function) $$\delta(t)$$ as the limit of a sequence of functions $$\{\delta_k(t)\}$$, where $$\int_{-\infty}^\infty\delta_k(t)dt=1$$For example,$$\delta_k(t)=\begin{cases}\frac k2,&|t|<1/k\\0,&\text{otherwise}\end{cases}$$is one such sequence. Then it goes on to say that the Dirac $$\delta$$ function can be understood as the generalized function$$\delta(t)=\begin{cases}0,&t\ne0\\\infty,&t=0\end{cases}$$For continuous functions in $$[0,\infty)$$ and $$a\ge0$$,$$\int_0^\infty f(t)\delta(t-a)=f(a)$$

• What is your definition of $\delta$? (for discontinuous functions, I mean) – Klaus Feb 19 at 13:59
• I'm not sure whether the initial integral is well defined in distribution theory. Suppose you modify it slightly so that the limit of $f$ for $x \rightarrow 1$ differs for $x>1$ and $x < 1$. What value do you expect to see? A discontinuous function is not a suitable test function as far as I know. – quarague Feb 19 at 14:07
• I don't see how you can apply $\delta$ to that function since it's not in $C^{\infty}$? – Drefain Feb 19 at 14:22
• @Klaus Please check the edit – Shubham Johri Feb 19 at 15:30
• Which text book are you using? Defining $\delta$ as a limit of functions seems shady to me and your example shows how this runs into trouble. Looking at the wikipedia article, if you define it through measure theory, you get $f(1)$ as the answer immediately per definition. If you define it via integrals with test functions which is what I was thinking about, this looks more complicated but the answer should be the same as the definitions are equivalent. – quarague Feb 19 at 15:54

The function $$f$$ has a removable discontinuity at $$1$$. Therefore, we have

$$\langle \delta_{1},f\rangle=\lim_{x\to 1}f(x)$$

To see this, we denote $$\delta_n(x)$$ as a regularization of the Dirac Delta, which for any suitable test function, $$\phi(x)$$, satisfies

$$\lim_{n\to \infty}\int_{-\infty}^\infty \delta_n(x)\phi(x)\,dx=\phi(0)$$

Now, suppose we have a function $$f(x)$$ that is of compact support and is smooth everywhere except at $$1$$ where it has a removable discontinuity. Let $$g(x)$$ be defined as

$$g(x)=\begin{cases}f(x)&,x\ne 1\\\\\lim_{x\to 1}f(x)&, x=1\end{cases}$$

Then, since $$g$$ is a suitable test functions we see that

\begin{align} \lim_{n\to \infty}\int_{-\infty}^\infty \delta_n(x-1)f(x)\,dx&=\lim_{n\to \infty}\int_{-\infty}^\infty \delta_n(x-1)g(x)\,dx\\\\ &=g(1)\\\\ &=\lim_{x\to 1}f(x) \end{align}

And we are done!

EXAMPLE:

As an example, suppose $$\delta_n(x)$$ is the asymmetrically centered pulse function, which for $$a\in(0,1)$$ given by

$$\delta_n(x)=\begin{cases}n&,x\in[-a/n,(1-a)/n]\\\\0&,\text{otherwise}\end{cases}$$

Then, we have

\begin{align} \lim_{n\to\infty}\int_{-\infty}^\infty \delta_n(x-1)f(x)\,dx&=\lim_{n\to\infty}\left(n\int_{1-a/n}^{1+(1-a)/n} f(x)\,dx\right)\\\\ &=\lim_{n\to\infty}\left(n\int_{1-a/n}^{1} f(x)\,dx\right)+\lim_{n\to\infty}\left(n\int_{1}^{1+(1-a)/n} f(x)\,dx\right)\\\\ &=a\lim_{x\to 1^-}f(x)+(1-a)\lim_{x\to 1^+}f(x)\\\\ &=\lim_{x\to 1}f(x) \end{align}

since the discontinuity at $$1$$ is removable and therefore the right-side and left-side limits are equal.

NOTE:

It is of paramount importance to understand that if $$f$$ has a removable discontinuity at $$x_0$$, then the functional $$\langle \delta_{x_0},f\rangle =\lim_{x\to x_0}f(x)$$ but if $$f$$ has a jump discontinuity at $$x_0$$, then the functional $$\langle \delta_{x_0},f\rangle$$ is not defined.

In fact, I showed in This Answer, that if $$H$$ is he Heaviside Function, then $$\langle \delta_0,H\rangle$$ is meaningless.

• Thank you for the detailed, well explained answer! This clears up a lot of things for me. – Shubham Johri Feb 23 at 7:23
• You're welcome. My pleasure. I'm pleased that this was helpful. – Mark Viola Feb 23 at 16:02