In functional analysis, a branch of mathematics, the Borel functional calculus is a functional calculus (that is, an assignment of operators from commutative algebras to functions defined on their spectra), which has particularly broad scope.[1][2] Thus for instance if T is an operator, applying the squaring function s → s2 to T yields the operator T2. Using the functional calculus for larger classes of functions, we can for example define rigorously the "square root" of the (negative) Laplacian operator −Δ or the exponential
The 'scope' here means the kind of function of an operator which is allowed. The Borel functional calculus is more general than the continuous functional calculus, and its focus is different than the holomorphic functional calculus.
More precisely, the Borel functional calculus allows for applying an arbitrary Borel function to a self-adjoint operator, in a way that generalizes applying a polynomial function.
^Kadison, Richard V.; Ringrose, John R. (1997). Fundamentals of the Theory of Operator Algebras: Vol 1. Amer Mathematical Society. ISBN 0-8218-0819-2.
^Reed, Michael; Simon, Barry (1981). Methods of Modern Mathematical Physics. Academic Press. ISBN 0-12-585050-6.
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