# Math Help - Denoting an interval

1. ## Denoting an interval

Hi

I have a position P determined by the point, at which a vector v points to. Now I have a function, which is only defined for the points on and inside a circle with radius 2 within P. Can I write this interval as

$
x,y \in (x-v_x)^2-(y-v_y)^2=4
$

?

Sincerely,
Niles.

2. That is a bit informal. You can write it like this:

$A := \{ (x,y) \in \mathbb{R} ^2 : \ (x - v_x)^2 - (y - v_y)^2 = 4 \}$

And then you can let $(x, y) \in A$ which, if I understand correctly is what you're looking for.