Permutations of Subscripts for Unchanged Value in Polynomial Symmetry

[polarbears]

Homework Statement

$$(x_1 - x_2)(x_3 - x_4)$$
Find permutations of subscripts that leave value unchanged

Homework Equations

The Attempt at a Solution

Okay so I know that it's asking how I should rearrange things and still not change the value.
Switching 1 with 2 or 3 with 4 would work but I know there's more than that but I'm don't know how to show it.
I guess I can expand it but I'm not sure if that helps

 

[Dick]

"Switching 1 with 2 OR 3 with 4" doesn't work, "switching 1 with 2 AND 3 with 4" does. How about switching 1 with 3 and 2 with 4? Hmm?