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Birkhoff–Kellogg invariant-direction theorem : ウィキペディア英語版 | Birkhoff–Kellogg invariant-direction theorem In functional analysis, the Birkhoff–Kellogg invariant-direction theorem, named after G. D. Birkhoff and O. D. Kellogg, is a generalization of the Brouwer fixed-point theorem. The theorem states that: Let ''U'' be a bounded open neighborhood of 0 in an infinite-dimensional normed linear space ''V'', and let ''F'':∂''U'' → ''V'' be a compact map satisfying ||''F''(''x'')|| ≥ α for some α > 0 for all ''x'' in ∂''U''. Then ''F'' has an invariant direction, ''i.e.'', there exist some ''x''o and some ''λ'' > 0 satisfying ''x''o = ''λF''(''x''o). The Birkhoff–Kellogg theorem and its generalizations by Schauder and Leray have applications to partial differential equations. ==References==
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