One of the fixed point theorems: If a completely-continuous operator $A$ maps a bounded closed convex set $K$ of a Banach space $X$ into itself, then there exists at least one point $x\in K$ such that $Ax=x$. Proved by J. Schauder  as a generalization of the Brouwer theorem.
There exist different generalizations of Schauder's theorem: the Markov–Kakutani theorem, Tikhonov's principle, etc.
|||J. Schauder, "Der Fixpunktsatz in Funktionalräumen" Stud. Math. , 2 (1930) pp. 171–180 Zbl 56.0355.01|
|||L.A. Lyusternik, V.I. Sobolev, "Elements of functional analysis" , Hindushtan Publ. Comp. (1974) (Translated from Russian) MR0539144 MR0048693 Zbl 0141.11601 Zbl 0096.07802|
|||N. Dunford, J.T. Schwartz, "Linear operators. General theory" , 1 , Interscience (1958) MR0117523|
|||R.E. Edwards, "Functional analysis: theory and applications" , Holt, Rinehart & Winston (1965) MR0221256 Zbl 0182.16101|
|||L. Nirenberg, "Topics in nonlinear functional analysis" , New York Univ. Inst. Math. Mech. (1974) MR0488102 Zbl 0286.47037|
The Tikhonov fixed-point theorem (also spelled Tychonoff's fixed-point theorem) states the following. Let be a locally convex topological space whose topology is defined by a family of continuous semi-norms. Let be compact and convex and a continuous mapping. Then has a fixed point in ([a2]; [a3], p. 175). Both the Kakutani fixed-point theorem and the Markov fixed-point theorem are generalized in the Ryll-Nardzewski fixed-point theorem, which states: Let be a Banach space and a non-empty weakly compact subset. Let be a semi-group of mappings from to which is non-contracting, then there is a fixed point of . Here, a family of mappings is said to have a fixed point if for every , , [a4]; cf. [a3], Chapt. 9, for a discussion of the Ryll-Nardzewski fixed-point theorem in relation to the Kakutani and Markov ones and other fixed-point theorems for families of mappings.
|[a1]||J. Dugundji, A. Granas, "Fixed-point theory" , I , PWN (1982) MR0660439 Zbl 0483.47038|
|[a2]||A.N. [A.N. Tikhonov] Tychonoff, "Ein Fixpunktsatz" Math. Ann. , 111 (1935) pp. 767–776 MR1513031 Zbl 0012.30803 Zbl 61.1195.01|
|[a3]||V.I. Istrăţescu, "Fixed point theory" , Reidel (1981) MR0620639 Zbl 0465.47035|
|[a4]||C. Ryll-Nardzewski, "On fixed points of semi-groups of endomorphisms of linear spaces" , Proc. 5-th Berkeley Symp. Probab. Math. Stat. , 2: 1 , Univ. California Press (1967) pp. 55–61|
Schauder theorem. Encyclopedia of Mathematics. URL: http://www.encyclopediaofmath.org/index.php?title=Schauder_theorem&oldid=29187