The paper introduces 4-fold symmetric quandles and 4-fold symmetric quandle homotopy invariants of 3-manifolds. We classify 4-fold symmetric quandles and investigate their properties. When the quandle is finite, we explicitly determine a presentation of its inner automorphism group. We calculate the container of the 4-fold symmetric quandle homotopy invariant. We also discuss symmetric quandle cocycle invariants and coloring polynomials of 4-fold symmetric quandles. 57M12, 57M25, 57M27, 57N70, 58K65; 55Q52, 22A30, 11E57, 55R40, 05E15
IntroductionA quandle is a set with a certain binary operation satisfying a self-distribute law. Quandles are adapted to the oriented link theory. For an oriented link L S 3 , Joyce [16] defined the link quandle Q L as an analog of the fundamental group 1 .S 3 n L/. For a quandle X , a quandle homomorphism Q L ! X is called an X -coloring of L. From algebraic topology, given a quandle X , Fenn, Rourke and Sanderson [9] defined the rack space analogous to the classifying space of groups. They [10; 11] show that the second homotopy group is isomorphic to a bordism group consisting of all "framed X -colorings". Then, a quandle homotopy invariant of oriented links can be defined by an invariant valued in the group ring ZOE 2 .BX /, where the space BX is a certain modification of the rack space. On the other hand, quandle cocycle invariants of oriented links introduced by Carter et al [3] using 2-cocycles of H 2 .BX I A/ are computable and practical; they can, however, be derived from the quandle homotopy invariant (see, eg, Carter, Kamada and Saito [4] and Fenn and Rourke [8]).In another direction, Hatakenaka [13] reformulated certain Dijkgraaf-Witten invariants of 3-manifolds [5] as quandle cocycle invariants. To see this, she made use of the fact that any 3-manifold is a 4-fold simple branched covering of the 3-sphere branched along a link L. Then the associated simple monodromy representation onto S 4 can be regarded as an S -coloring of L, which we call a labeled link. Here S WD f.ij / 2 S 4 g is a quandle with the conjugate operation. Hence, we may consider any 3-manifold to be a labeled link. Further, it is known (see Apostolakis [1] and Bobtcheva and Piergallini [2]) that homeomorphism classes of 3-manifolds are in 1-1 correspondence with the set of labeled links modulo some "MI and MII moves" (see Figure 3). The key point is that, using these facts, she presented the Dijkgraaf-Witten invariants of 3-manifolds as some invariants of labeled links.In this paper, our purpose is to construct and study an invariant of 3-manifolds obtained from the quandle homotopy invariant of labeled links. The idea behind the construction is simple: since any 3-manifold can be regarded as a labeled link of L, we define a quandle X over S , and consider X -colorings obtained by lifting the S -coloring to be an invariant of the 3-manifold. For this, noting that the monodromies are unrelated to orientations of links, we focus on symmetric quandles introduced by Kamada [17] which are suitable for un...