The Nielsen realization problem is a question asked by Jakob Nielsen (1932, pp. 147–148) about whether finite subgroups of mapping class groups can act on surfaces, that was answered positively by Steven Kerckhoff (1980, 1983).

Given an oriented surface, we can divide the group Diff(S), the group of diffeomorphisms of the surface to itself, into isotopy classes to get the mapping class group π₀(Diff(S)). The conjecture asks whether a finite subgroup of the mapping class group of a surface can be realized as the isometry group of a hyperbolic metric on the surface.

The mapping class group acts on Teichmüller space. An equivalent way of stating the question asks whether every finite subgroup of the mapping class group fixes some point of Teichmüller space.

Jakob Nielsen (1932, pp. 147–148) asked whether finite subgroups of mapping class groups can act on surfaces. Kravetz (1959) claimed to solve the Nielsen realization problem but his proof depended on trying to show that Teichmüller space (with the Teichmüller metric) is negatively curved. Linch (1971) pointed out a gap in the argument, and Masur (1975) showed that Teichmüller space is not negatively curved. Steven Kerckhoff (1980, 1983) gave a correct proof that finite subgroups of mapping class groups can act on surfaces using left earthquakes.

• Kerckhoff, Steven P. (1980), "The Nielsen realization problem" (PDF), Bulletin of the American Mathematical Society, New Series, 2 (3): 452–454, doi:10.1090/S0273-0979-1980-14764-3, ISSN 0002-9904, MR 0561531
• Kerckhoff, Steven P. (1983), "The Nielsen realization problem", Annals of Mathematics, Second Series, 117 (2): 235–265, CiteSeerX 10.1.1.353.3593, doi:10.2307/2007076, ISSN 0003-486X, JSTOR 2007076, MR 0690845
• Kravetz, Saul (1959), "On the geometry of Teichmüller spaces and the structure of their modular groups", Ann. Acad. Sci. Fenn. Ser. A I, 278: 35, MR 0148906
• Linch, Michele Regina (1971), ON METRICS IN TEICHMUELLER SPACE, MR 2620985 – via ProQuest
• Masur, Howard (1975), "On a class of geodesics in Teichmüller space", Annals of Mathematics, Second Series, 102 (2): 205–221, doi:10.2307/1971031, ISSN 0003-486X, JSTOR 1971031, MR 0385173
• Nielsen, Jakob (1932), "Untersuchungen zur Topologie der geschlossenen zweiseitigen Flächen. III.", Acta Math. (in German), 58 (1): 87–167, doi:10.1007/BF02547775, Zbl 0004.27501