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124 Chapter 4 Spectral Analysis<br />

The next proposition shows that In(λ) can be regarded as a sample analogue of<br />

2πf (λ). Recall that if ∞ h−∞ |γ (h)| < ∞, then<br />

2πf (λ) <br />

∞<br />

γ (h)e −ihλ , λ ∈ (−π, π]. (4.2.8)<br />

h−∞<br />

Proposition 4.2.1 If x1,...,xnare any real numbers and ωk is any of the nonzero Fourier frequencies<br />

2πk/nin ( − π,π], then<br />

In(ωk) <br />

ˆγ (h)e −ihωk , (4.2.9)<br />

|h|

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