# Change of integral variable

1. Aug 11, 2016

### baby_1

1. The problem statement, all variables and given/known data
I want to change integral variable to jx it means (w=jx)

2. Relevant equations

3. The attempt at a solution

but as you see the bounds of integral are different from the book text , what is my mistake?

2. Aug 13, 2016

### Simon Bridge

hat about the part that says: "by using the Cauchy theorem and showing that..."?

3. Aug 14, 2016

### baby_1

dear Simon
I didn't understand how can I use the Cauchy tip to solve this inconsistency?

4. Aug 14, 2016

### Simon Bridge

It's a step you seemto have left out though.
I'll agree it looks like there's a minus sign missing. Maybe there's a typo? I can't actually read the image properly.

5. Aug 14, 2016

### pasmith

It allows you to perform the integral from $\omega = -i\infty$ to $\omega = +i\infty$, which on setting $\omega = ix$ becomes $x = -\infty$ to $x = \infty$.