In mathematical analysis, and especially functional analysis, a fundamental role is played by the space of continuous functions on a compact Hausdorff space X {\displaystyle X} with values in the real or complex numbers. This space, denoted by C ( X ) , {\displaystyle {\mathcal {C}}(X),} is a vector space with respect to the pointwise addition of functions and scalar multiplication by constants. It is, moreover, a normed space with norm defined by
‖ f ‖ = sup x ∈ X | f ( x ) | , {\displaystyle \|f\|=\sup _{x\in X}|f(x)|,}
the uniform norm. The uniform norm defines the topology of uniform convergence of functions on X . {\displaystyle X.} The space C ( X ) {\displaystyle {\mathcal {C}}(X)} is a Banach algebra with respect to this norm.(Rudin 1991, §10.3(a))
Properties By Urysohn's lemma, C ( X ) {\displaystyle {\mathcal {C}}(X)} separates points of X {\displaystyle X} : If x , y ∈ X {\displaystyle x,y\in X} are distinct points, then there is an f ∈ C ( X ) {\displaystyle f\in {\mathcal {C}}(X)} such that f ( x ) ≠ f ( y ) . {\displaystyle f(x)\neq f(y).}
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