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Use cycles-per-time spectral density, with frequency $\omega \in [-1/2,1/2]$. The spectral representation theorem for a stationary time series gives the centered white noise representationwhere disjoint increments are orthogonal. Put ${\theta }_0=1$. Since the filter is finite, substitute each noise representation and interchange the finite sum with the integral:The new orthogonal increment measure is $d{Z}_X=\Theta (e^{-2\pi i\omega })d{Z}_{\epsilon }$. Its variance measure is therefore ${\sigma }^2|\Theta (e^{-2\pi i\omega }){|}^{2}d\omega$. The coefficients are real, so conjugation changes the sign of the exponent without changing the modulus. HenceWith angular frequency $\lambda =2\pi \omega$, the spectral density of a stationary process instead contains the factor $1/(2\pi )$. The convention explains its absence here. Invertibility is not needed for this finite-filter spectral calculation.