Lower bounds for regular genus and gem-complexity of PL 4-manifolds

Within crystallization theory, two interesting PL invariants for $d$-manifolds have been introduced and studied, namely {\it gem-complexity} and {\it regular genus}. In the present paper we prove that, for any closed connected PL $4$-manifold $M$, its gem-complexity $\mathit{k}(M)$ and its regular genus $\mathcal G(M)$ satisfy: $$\mathit{k}(M) \ \geq \ 3 \chi (M) + 10m -6 \ \ \ \text{and} \ \ \ \mathcal G(M) \ \geq \ 2 \chi (M) + 5m -4,$$ where $rk(\pi_1(M))=m.$ These lower bounds enable to strictly improve previously known estimations for regular genus and gem-complexity of product 4-manifolds. Moreover, the class of {\it semi-simple crystallizations} is introduced, so that the represented PL 4-manifolds attain the above lower bounds. The additivity of both gem-complexity and regular genus with respect to connected sum is also proved for such a class of PL 4-manifolds, which comprehends all ones of "standard type", involved in existing crystallization catalogues, and their connected sums.Comment: 17 pages, 3 figures. To appear in Forum Mathematicu

A note about complexity of lens spaces

Within crystallization theory, (Matveev's) complexity of a 3-manifold can be estimated by means of the combinatorial notion of GM-complexity. In this paper, we prove that the GM-complexity of any lens space L(p,q), with p greater than 2, is bounded by S(p,q)-3, where S(p,q) denotes the sum of all partial quotients in the expansion of q/p as a regular continued fraction. The above upper bound had been already established with regard to complexity; its sharpness was conjectured by Matveev himself and has been recently proved for some infinite families of lens spaces by Jaco, Rubinstein and Tillmann. As a consequence, infinite classes of 3-manifolds turn out to exist, where complexity and GM-complexity coincide. Moreover, we present and briefly analyze results arising from crystallization catalogues up to order 32, which prompt us to conjecture, for any lens space L(p,q) with p greater than 2, the following relation: k(L(p,q)) = 5 + 2 c(L(p,q)), where c(M) denotes the complexity of a 3-manifold M and k(M)+1 is half the minimum order of a crystallization of M.Comment: 14 pages, 2 figures; v2: we improved the paper (changes in Proposition 10; Corollary 9 and Proposition 11 added) taking into account Theorem 2.6 of arxiv:1310.1991v1 which makes use of our Prop. 6(b) (arxiv:1309.5728v1). Minor changes have been done, too, in particular to make references more essentia

An equivalence criterion for PL-manifolds

Aim of the paper is to translate the homeomorphism problem for n-dimensional PL-manifolds, with or without boundary, into an equivalence problem for pseudosimplicial triangulations (i.e. a suitable generalization of simplicial triangulations, where two (curvilinear) simplices may intersect in more than one face), by means of a finite number of moves, called (geometric) dipole moves. Note that the environment of the present work is closely related to the representation method for PL-manifolds via edge-colorured graphs, since (n+1)-coloured graphs representing n-manifolds are a «discrete way» to visualize suitable pseudosimplicial triangulations. From the graphtheoretical point of view, the equivalence problem was already faced – and solved – in [FG] for closed n-manifolds and in [C2] in the general 3-dimensional setting; here, the equivalence criterion via (geometric) dipole moves is proved to hold for the whole class of PL n-manifolds; moreover, it is proved to be equivariant with respect to boundary triangulation

Average order of coloured triangulations : the general case

In [Combinatorics of triangulations of 3-manifolds, Trans. Amer. Math. Soc. 337 (2) (1993), 891-906], Luo and Stong introduced the notion of "average edge order" \mu_0(K) = \frac {3 F_0(K)}{E_0(K), K being a triangulation of a closed 3-manifold M with $E_0(K)$ edges and $F_0(K)$ triangles. The present paper extends the above notion to the "average (n-2)-simplex order" of a coloured triangulation K of a compact PL n-manifold $M^n$ with $\alpha_i(K)$ i-simplices: \mu(K) = \frac {n \alpha_{n-1}(K)}{\alpha_{n-2}(K).Main properties of $\mu(K)$ and its relations with the topology of $M^n$, both in the closed and bounded case, are investigated; the obtained results show the existence of strong analogies with the 3-dimensional simplicial case (see the quoted paper by Luo and Stong, together with [The average edge order of triangulations of 3-manifolds, Osaka J. Math. 33(1986), 761-773] by Tamura)

A Code for m-Bipartite Edge-Coloured Graphs

An (n + 1)-coloured graph $\left(\Gamma,\gamma\right)$ is said to be $m-bipartite$ if m is the maximum integer so that every m-residue of $\left(\Gamma,\gamma\right)$ (i.e. every connected subgraph whose edges are coloured by only m colours) is bipartite; obviously, every (n + 1)-coloured graph, with n $\geq$ 2, results to be m-bipartite for some m, with 2 $\leq$ m $\leq$ n + 1. In this paper, a numerical $code$ of length (2n \textminus{} m + 1) $\times$ q is assigned to each m-bipartite (n + 1)-coloured graph of order 2q. Then, it is proved that$any\; two\; such\; graphs\; have\; the\; same\; code\; if\; and\; only\; if\; they\; are\; colour-isomorphic$, i.e. if a graph isomorphism exists, which transforms the graphs one into the other, up to permutation of the edge-colouring. More precisely, if H is a given group of permutations on the colour set, we face the problem of algorithmically recognizing H-isomorphic coloured graphs by means of a suitable defi{}nition of H-code

A catalogue of orientable 3-manifolds triangulated by 30 coloured tetrahedra

The present paper follows the computational approach to 3-manifold classification via edge-coloured graphs, already performed in [1] (with respect to orientable 3-manifolds up to 28 coloured tetrahedra), in [2] (with respect to non-orientable3-manifolds up to 26 coloured tetrahedra), in [3] and [4] (with respect to genus two 3-manifolds up to 34 coloured tetrahedra): in fact, by automatic generation and analysis of suitable edge-coloured graphs, called crystallizations, we obtain a catalogue of all orientable 3-manifolds admitting coloured triangulations with 30 tetrahedra. These manifolds are unambiguously identified via JSJ decompositions and fibering structures. It is worth noting that, in the present work, a suitable use of elementary combinatorial moves yields an automatic partition of the elements of the generated crystallization catalogue into equivalence classes, which turn out to be in one-to one correspondence with the homeomorphism classes of the represented manifolds

Gem-induced trisections of compact PL 44-manifolds

The idea of studying trisections of closed smooth $4$-manifolds via (singular) triangulations, endowed with a suitable vertex-labelling by three colors, is due to Bell, Hass, Rubinstein and Tillmann, and has been applied by Spreer and Tillmann to colored triangulations associated to the so called simple crystallizations of standard simply-connected $4$-manifolds. The present paper performs a generalization of these ideas along two different directions: first, we take in consideration also compact PL $4$-manifolds with connected boundary, introducing a possible extension of trisections to the boundary case; then, we analyze the trisections induced not only by simple crystallizations, but by any 5-colored graph encoding a simply-connected $4$-manifold. This extended notion is referred to as gem-induced trisection, and gives rise to the G-trisection genus, generalizing the well-known trisection genus. Both in the closed and boundary case, we give conditions on a 5-colored graph which ensure one of its gem-induced trisections - if any - to realize the G-trisection genus, and prove how to determine it directly from the graph itself. Moreover, the existence of gem-induced trisections and an estimation of the G-trisection genus via surgery description is obtained, for each compact simply-connected PL 4-manifold admitting a handle decomposition lacking in 1-handles and 3-handles. As a consequence, we prove that the G-trisection genus equals $1$ for all $\mathbb D^2$-bundles of $\mathbb S^2$, and hence it is not finite-to-one.Comment: 25 pages, 14 figures. Updated to most recent versio

Kirby diagrams and 5-colored graphs representing compact 4-manifolds

It is well-known that any framed link $(L,c)$ uniquely represents the 3-manifold $M^3(L,c)$ obtained from $\mathbb S^3$ by Dehn surgery along $(L,c)$, as well as the PL 4-manifold $M^4(L,c)$ obtained from $\mathbb D^4$ by adding 2-handles along $(L,c)$, whose boundary coincides with $M^3(L,c)$. In this paper we study the relationships between the above representation tool in dimension 3 and 4, and the representation theory of compact PL manifolds of arbitrary dimension by edge-coloured graphs: in particular, we describe how to construct a (regular) 5-colored graph representing $M^4(L,c)$, directly "drawn over" a planar diagram of $(L,c)$. As a consequence, the combinatorial properties of the framed link $(L,c)$ yield upper bounds for both the invariants gem-complexity and (generalized) regular genus of $M^4(L,c)$.Comment: 16 pages, 15 figures. Figures 9 and 10 have been corrected. arXiv admin note: text overlap with arXiv:1910.0877

Complexity computation for compact 3-manifolds via crystallizations and Heegaard diagrams

The idea of computing Matveev complexity by using Heegaard decompositions has been recently developed by two different approaches: the first one for closed 3-manifolds via crystallization theory, yielding the notion of Gem-Matveev complexity; the other one for compact orientable 3-manifolds via generalized Heegaard diagrams, yielding the notion of modified Heegaard complexity. In this paper we extend to the non-orientable case the definition of modified Heegaard complexity and prove that for closed 3-manifolds Gem-Matveev complexity and modified Heegaard complexity coincide. Hence, they turn out to be useful different tools to compute the same upper bound for Matveev complexity.Comment: 12 pages; accepted for publication in Topology and Its Applications, volume containing Proceedings of Prague Toposym 201

Computing Matveev's complexity via crystallization theory: the boundary case

The notion of Gem-Matveev complexity has been introduced within crystallization theory, as a combinatorial method to estimate Matveev's complexity of closed 3-manifolds; it yielded upper bounds for interesting classes of such manifolds. In this paper we extend the definition to the case of non-empty boundary and prove that for each compact irreducible and boundary-irreducible 3-manifold it coincides with the modified Heegaard complexity introduced by Cattabriga, Mulazzani and Vesnin. Moreover, via Gem-Matveev complexity, we obtain an estimation of Matveev's complexity for all Seifert 3-manifolds with base $\mathbb D^2$ and two exceptional fibers and, therefore, for all torus knot complements.Comment: 27 pages, 14 figure