Fourier transform of a triangular function
= Fourier transform of a triangular function
For $\theta_n(x)=(1-|x|/n)_+$,
$$
\widehat\theta_n(\xi)
=\frac{2(1-\cos(n\xi))}{n\xi^2}
=n\left(\frac{\sin(n\xi/2)}{n\xi/2}\right)^2,
$$
with value $n$ at zero.