###### Answer: A. (f+g)(x) ≥ 3 for all values of x

Let f be a function whose domain is the set X and whose codomain is the set Y.Then f is invertible if there exists a function g with domain Y and codomain X with the property: = ⇔ =. If f is invertible then the function g is unique which means that there is exactly one function g satisfying this property. Moreover it also follows that the ranges of g and f equal their respective codomains.

More formally f = g if f ( x ) = g ( x ) for all x ∈ X where f : X → Y and g : X → Y. [8] [ 9 ] [note 4] The domain and codomain are not always explicitly given when a function is defined and without some (possibly difficult) computation one might only know that the domain is contained in a larger set.

Eq.1) The function S ¯ x x ( f ) {\displaystyle {\bar {S}}_{xx}( f )} and the autocorrelation of x (t) {\displaystyle x (t)} form a Fourier transform pair a result is known as Wiener–Khinchin theorem . (also see Periodo…