Which of these is a correct definition of Fourier transform?
$$\mathcal{F}(f(x))= \int\limits_{-\infty}^{\infty} f(x)\, e^{-i k x} \, \mathrm{d} x \ \ \ \ (1)$$ and$$\mathcal{F}(f(x))= \int\limits_{-\infty}^{\infty} f(x)\, e^{i k x} \, \mathrm{d} x\ \ \ \ (2)$$
I use form (1) with a negative exponent,but I see some people use form (2). I want to know which is the correct definition.