Difine a first order formula for positive numbers

Let be the vocabulary of arithmetic. Let R be the structure that has universe and interprets the vocabulary in the usual manner.

Define a formula such that for any

if and only if a is positive.

Im sure I really should be able to do this but I cannot think how to.

Thanks for any help

For clarity is the structure and is the real numbers.

(what is the LaTex for the reals?)

Thanks again

Re: Difine a first order formula for positive numbers

How about ?

Re: Difine a first order formula for positive numbers

haha yeah thanks I was thinking about that for ages!