Abstract. We show that for any orientation-preserving self-homeomorphism α of the double torus Σ 2 there exists a point p of Σ 2 such that α(α(p)) = p. This answers a question raised by Jakob Nielsen in 1942.
BackgroundThroughout this article, R will denote a commutative ring, and g a positive integer. We shall write Σ g to denote the closed, connected, orientable surface of genus g, and Z g to denote the ring Z/gZ.Nielsen [10] (cf. [7]) showed that, for any g ≥ 2, there exists an orientationpreserving self-homeomorphism α of Σ g such that α, α 2 , . . . , α 2g−3 are all fixedpoint free, that is, have no fixed-points. He showed further that, for any orientationpreserving self-homeomorphism α of Σ g , at least one of α, α 2 , . . . , α 2g−3 , α 2g−2 has a fixed-point if g ≥ 3, and at least one of α, α 2 , α 3 has a fixed-point if g = 2.Since 2g − 2 = 2 for g = 2, this left open the question of exactly what happens in the case g = 2, and he commented that it seemed difficult to him [10, Section 4] (cf. [7]). The problem has not been forgotten; for example, in a recent paper in which he obtains results analogous to Nielsen's for orientation-reversing selfhomeomorphisms and for non-orientable surfaces, Wang [13] mentions that it is still open. In the next section we solve this problem, rounding off Nielsen's abovementioned results by showing that, for g = 2, at least one of α, α 2 has a fixed-point, or, equivalently, α 2 has a fixed-point.This result has its origins in classical topology, has connections with dynamical systems, and has a proof which is mainly algebraic. Let us review the basic information that will be used.Recall that the fundamental group of Σ g has a one-relator presentationwhere [x, y] denotes xyx −1 y −1 . Recall also that the first homology R-module of Σ g , denoted H 1 (Σ g , R), is the R-module obtained by abelianizing the fundamental group and then tensoring over Z with R. There is a natural group homomorphism