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In functional analysis, the Hille–Yosida theorem characterizes the generators of strongly continuous one-parameter semigroups of linear operators on Banach spaces. It is sometimes stated for the special case of contraction semigroups, with the general case being called the Feller–Miyadera–Phillips theorem (after William Feller, Isao Miyadera, and Ralph Phillips). The contraction semigroup case is widely used in the theory of Markov processes. In other scenarios, the closely related Lumer–Phillips theorem is often more useful in determining whether a given operator generates a strongly continuous contraction semigroup. The theorem is named after the mathematicians Einar Hille and Kōsaku Yosida who independently discovered the result around 1948. == Formal definitions == (詳細はone-parameter semigroup of operators on ''X'' is a family of operators indexed on the non-negative real numbers ''t'' ∈ * * The semigroup is said to be strongly continuous, also called a (''C''0) semigroup, if and only if the mapping : is continuous for all ''x'' ∈ ''X'', where The infinitesimal generator of a one-parameter semigroup ''T'' is an operator ''A'' defined on a possibly proper subspace of ''X'' as follows: *The domain of ''A'' is the set of ''x'' ∈ ''X'' such that :: :has a limit as ''h'' approaches 0 from the right. * The value of ''A'' x is the value of the above limit. In other words ''A'' ''x'' is the right-derivative at 0 of the function :: The infinitesimal generator of a strongly continuous one-parameter semigroup is a closed linear operator defined on a dense linear subspace of ''X''. The Hille–Yosida theorem provides a necessary and sufficient condition for a closed linear operator ''A'' on a Banach space to be the infinitesimal generator of a strongly continuous one-parameter semigroup. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Hille–Yosida theorem」の詳細全文を読む スポンサード リンク
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