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Theory of Statistics - George Mason University

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0.1 Measure, Integration, and Functional Analysis 733<br />

f ◦ g(x) = f(g(x)). (0.1.68)<br />

Notice that the operation is not commutative and that the range <strong>of</strong> g must<br />

be a subset <strong>of</strong> the domain <strong>of</strong> f for the composition to exist.<br />

Two useful types <strong>of</strong> real function spaces are those that contain smooth<br />

functions and those that contain integrable functions. We first describe spaces<br />

<strong>of</strong> smooth functions <strong>of</strong> various degrees, and then in Section 0.1.9 discuss spaces<br />

<strong>of</strong> integrable functions <strong>of</strong> various types and the kinds <strong>of</strong> operations that can<br />

be defined on those spaces.<br />

Other useful operations on functions are correlation, convolution, inner<br />

product, and other transforms. Whether or not a given type <strong>of</strong> operation can<br />

be defined on a function space may depend on the properties <strong>of</strong> functions, in<br />

particular, the integrability <strong>of</strong> the functions. We therefore defer discussion <strong>of</strong><br />

these other operations to Section 0.1.9.<br />

Spaces <strong>of</strong> Smooth Real Functions<br />

Differentiability is a smoothness property.<br />

Definition 0.1.41 (C k space)<br />

For an integer k ≥ 0, a function f such that all derivatives up to the k th<br />

derivative exist and are continuous is said to belong to the class C k .<br />

The notation C k does not specify the domain <strong>of</strong> the functions. Generally,<br />

without any further notation, for d-variate functions, the domain is taken to<br />

be IR d . A domain D can be specified by the notation C k (D). For example,<br />

C 0 ([0, 1]) refers to the class <strong>of</strong> all continuous functions over the unit interval<br />

[0, 1].<br />

The class C 0 includes all continuous functions.<br />

If f ∈ C k then f ∈ C j for j ≤ k.<br />

A C k class <strong>of</strong> functions over the same domain is a linear space (exercise).<br />

The term “smooth” is used in connection with the C k classes. In a relative<br />

sense, for j < k, a function in C k is smoother than one in C j and not in C k .<br />

In an absolute sense, a function is said to be “smooth” if it is in C ∞ .<br />

Analytic Functions<br />

Not all functions in C ∞ have a convergent Taylor series at any point (see<br />

page 650). The special ones that do are said to be analytic over the region in<br />

which the Taylor series at the point x0 converges to the value <strong>of</strong> the function<br />

at x0. (The Taylor series may not converge, and more remarkably, it may<br />

not converge to the value <strong>of</strong> the function.) We sometimes denote the class<br />

<strong>of</strong> analytic functions as C ω . Analytic functions are <strong>of</strong> course smooth, and<br />

C ω ⊆ C ∞ .<br />

<strong>Theory</strong> <strong>of</strong> <strong>Statistics</strong> c○2000–2013 James E. Gentle

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