Which of the following functions are suitable auto correlation functions? Tell why or why not. ( 0

Which of the following functions are suitable auto correlation functions? Tell why or why not. (ω₀, τ₀, τ₁, A, B, C, and f₀ are positive constants.)

(a) A cos ω₀τ

(b) AΛ (τ/ τ₀), where Λ(x) is the unit-area triangular function defined in Chapter 2

(c) AII (τ/ τ₀), where II(x) is the unit-area pulse function defined in Chapter 2

(d) A exp (-τ / τ₀) u(τ) where u(x) is the unit-step function

(e) A exp(-|τ|/ τ₀)

(f) A sinc (f₀τ) sin(πf₀τ) / πf₀τ