# One hard integral

## Homework Statement

Solve

$$\int^{\infty}_0\frac{\sin x}{\sqrt{x}}$$

## The Attempt at a Solution

$$\lim_{t\to \infty}\int^{t}_0\frac{\sin x}{\sqrt{x}}$$
$$\sqrt{x}=v$$

so

$$\lim_{t\to \infty}\int^{t}_0\frac{\sin v^2}{v}$$

Integration by parts maybe? What is idea?

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ehild
Homework Helper
Do not forget dx from the integral. When substituting, you need to transform dx too.

ehild

Mistake

I will get

$$2\int^{\infty}_0\sin v^2dv$$

SammyS
Staff Emeritus
Homework Helper
Gold Member
Mistake

I will get

$$2\int^{\infty}_0\sin v^2dv$$
You will get $\displaystyle \int^{t}_0\frac{\sin x}{\sqrt{x}}\,dx=2\int^{\sqrt{t}}_0\sin (v^2)\,dv\,.$

You will get $\displaystyle \int^{t}_0\frac{\sin x}{\sqrt{x}}\,dx=2\int^{\sqrt{t}}_0\sin (v^2)\,dv\,.$

But t goes into $$\infty$$ so I write in correct way last step.

Have you been introduced to the idea of using contours in the complex plane to solve integrals on the real line? Because if you have, that seems like the simplest way to solve this.

Is there some other way?

HallsofIvy
Homework Helper
No, not every problem has a trivial solution!

Is there some other way?

Nope, have you been introduced to the idea I mentioned? I find it hard to believe that this question would be set if you haven't...

lurflurf
Homework Helper
$$\int^{\infty}_0\frac{\sin x}{\sqrt{x}}=\lim_{a\rightarrow 0^+}\int^{\infty}_0 \frac{\sin x}{\sqrt{x}}e^{-a x}=\lim_{a\rightarrow 0^+}\sqrt{\frac{\pi}{2(a^2+1)(a+\sqrt{a^2+1})}}$$

Complex methods are not the only way to do integrals.
or
The complex part of complex methods to do integrals can be hidden.
Depending on ones perspective.

I like Serena
Homework Helper
Nope, have you been introduced to the idea I mentioned? I find it hard to believe that this question would be set if you haven't...

Ok. But I would like to see some trick how to calculate this integral in the desert island :)

I would expect you to have a table of standard integrals or something in which this one is included.

$$\int^{\infty}_0\frac{\sin x}{\sqrt{x}}=\lim_{a\rightarrow 0^+}\int^{\infty}_0 \frac{\sin x}{\sqrt{x}}e^{-a x}=\lim_{a\rightarrow 0^+}\sqrt{\frac{\pi}{2(a^2+1)(a+\sqrt{a^2+1})}}$$

Complex methods are not the only way to do integrals.
or
The complex part of complex methods to do integrals can be hidden.
Depending on ones perspective.

And how you get

$$\int^{\infty}_0 \frac{\sin x}{\sqrt{x}}e^{-a x}dx=\sqrt{\frac{\pi}{2(a^2+1)(a+\sqrt{a^2+1})}}$$

I like Serena
Homework Helper
$$\int^{\infty}_0\frac{\sin x}{\sqrt{x}}=\lim_{a\rightarrow 0^+}\int^{\infty}_0 \frac{\sin x}{\sqrt{x}}e^{-a x}=\lim_{a\rightarrow 0^+}\sqrt{\frac{\pi}{2(a^2+1)(a+\sqrt{a^2+1})}}$$

Complex methods are not the only way to do integrals.
or
The complex part of complex methods to do integrals can be hidden.
Depending on ones perspective.

Looks like a Laplace transform.
Seems to me that it's not trivial to calculate it yourself.
Did you use some table to find the transform?

lurflurf
Homework Helper
We want to get rid of the 1/sqrt(t) so substitute
$$\frac{1}{\sqrt{t}}=\frac{2}{\sqrt{\pi}}\int_0^{ \infty } e^{-s^2 t}ds$$
then interchange s and t integrations
$$\int^{\infty}_0 \frac{\sin(b t)}{\sqrt{t}}e^{-a t}dt=\frac{2}{\sqrt{\pi}}\int_0^{ \infty }\left\{\int_0^{\infty}\sin(b t)e^{-a t}e^{-s^2 t}dt \right\}ds=\frac{2}{\sqrt{\pi}}\int_0^{ \infty } \frac{b}{(s^2+a)^2+b^2} ds=b \cdot \sqrt{\frac{\pi}{2} \cdot \frac{1}{(a^2+b^2)(a+\sqrt{a^2+b^2})}}$$
$$\int^{\infty}_0 \frac{\cos(b t)}{\sqrt{t}}e^{-a t}dt= \frac{2}{\sqrt{\pi}}\int_0^{ \infty }\left\{\int_0^{\infty}\cos(b t)e^{-a t}e^{-s^2 t}dt \right\}ds=\frac{2}{\sqrt{\pi}}\int_0^{ \infty } \frac{s^2+a}{(s^2+a)^2+b^2} ds=\sqrt{\frac{\pi}{2} \cdot \frac{a+\sqrt{a^2+b^2}}{a^2+b^2}}$$

That last integral is messy, but it can be done with usual elementary calculus methods and functions. These might be in some Laplace transform tables though often some manipulation is required to get the right form.

Last edited:
dextercioby