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A Classification of Spanning Surfaces for Alternating Links

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A CLASSIFICATION OF SPANNING SURFACES FOR ALTERNATING LINKS COLIN ADAMS AND THOMAS KINDRED Abstract. A classification of spanning surfaces for alternating links is provided up to genus, orientability, and a new invariant that we call aggregate slope. That is, given an alternating link, we determine all possible combinations of genus, orientability, and aggregate slope that a surface spanning that link can have. To this end, we describe a straightforward algorithm, much like Seifert's Algorithm, through which to construct certain spanning surfaces called layered surfaces. A particularly important subset of these will be what we call basic layered surfaces. We can alter these surface by performing the entirely local operations of adding handles and/or crosscaps, each of which increases genus. The main result then shows that if we are given an alternating projection P (L) and a surface S spanning L, we can construct a surface T spanning L with the same genus, orientability, and aggregate slope as S that is a basic layered surface with respect to P , except perhaps at a collection of added crosscaps and/or handles. Furthermore, S must be connected if L is non-splittable. This result has several useful corollaries. In particular, it allows for the determination of nonorientable genus for alternating links. It also can be used to show that mutancy of alternating links preserves nonorientable genus. And it allows one to prove that there are knots that have a pair of minimal nonorientable genus spanning surfaces, one boundary-incompressible and one boundary-compressible. 1. Introduction A Seifert surface for an oriented knot or link L is a compact connected oriented surface in S3 with oriented boundary equal to L. Seifert's Algorithm, wherein one takes an oriented projection of the link, cuts each crossing open in the manner that preserves orientation, fills in each resulting circle with a disk, and connects the disks with a half-twisted band at each crossing, ensures that such a surface exists. The minimal genus of a Seifert surface is defined to be the genus of the knot or link. Given a random link, it is typically difficult to determine its genus. However for one category of link, we have a simple means for finding the genus. Theorem 1.1. Given an oriented alternating link, Seifert's Algorithm applied to a reduced alter- nating projection yields a minimal genus Seifert surface. arXiv:1205.5520v1 [math.GT] 24 May 2012 This theorem has been proven three times, first independently in 1958 by Kunio Murasugi [23] and in 1959 by Richard Crowell [9] using the reduced Alexander Polynomial, and then in 1987 by David Gabai [11] using elementary geometric techniques. Our main theorem will include this theorem as a special case. However, in this paper, we will also be concerned with nonorientable spanning surfaces. The cross- cap number of a nonorientable surface S of n boundary components is defined to be 2 − χ(S) − n. Note that this is the number of projective planes of which the corresponding closed surface is the connected sum. In [8], the crosscap number of a link L is defined to be the minimum crosscap number of any nonorientable surface spanning the link L. Date: May 25, 2012. 12000 Mathematics Subject Classification 57M25, 57M50 Key words: spanning surface, alternating knot, crosscap number 1 2 COLIN ADAMS AND THOMAS KINDRED For the sake of comparison between complexities of orientable and nonorientable surfaces, define the nonorientable genus of L to be 1/2 of the crosscap number of L. Note that this can be either an integer or a half integer, and does not coincide with traditional definitions. We will sometimes refer to the genus of a knot or link as the orientable genus if confusion may otherwise arise. The crosscap number is known only for a relatively small collection of knots and links. In [30], it was determined for torus knots (see also [24]). In [13], a simple method for its determination was provided for two-bridge knots and links. In [15], it was determined for many pretzel knots. In this paper, we present a classification of spanning surfaces for alternating links up to genus, orientability, and a new invariant that we call aggregate slope, and that for knots coincides with the slope. That is, given an alternating link, we determine all possible combinations of genus, ori- entability, and aggregate slope that a surface spanning that link can have. To this end, we describe a straightforward algorithm, much like Seifert's Algorithm, through which to construct spanning sur- faces. We call the surfaces generated by this algorithm layered surfaces. A particularly important subset of these will be what we call basic layered surfaces. Once we have a layered surface, we can also change the surface by performing the entirely local operations of adding handles and/or crosscaps, each of which increases genus. Our main result, which appears in Section 5, is as follows: Theorem 5.3. Given an alternating projection P (L) and a surface S spanning L, we can construct a surface T spanning L with the same genus, orientability, and aggregate slope as S such that T is a basic layered surface with respect to P , except perhaps at a collection of added crosscaps and/or handles. When S is orientable, T can be chosen to be orientable with respect to the orientation that L inherits from S. This result has several useful corollaries that appear in Section 6, including the already mentioned classification of spanning surfaces for alternating links. Additionally, it allows us to easily find and construct a minimal nonorientable genus surface spanning a given alternating link. Note, for example 2 in [33], that a substantial amount of work is necessary to show the crosscap number of the link 63 is 3. Theorem 5.4 and the algorithm given below make this an immediate conclusion. At the end of the paper, we include the nonorientable genus of all the alternating knots of nine or fewer crossings and alternating 2-component links of eight or fewer crossings. It would be straightforward to write a computer program to determine the nonorientable genus of all knots in the census of alternating knots through 22 crossings given in [14],[28] and [29]. In [13], Hatcher and Thurston proved that a 2-bridge knot cannot have two minimal nonorientable genus spanning surfaces, one boundary-incompressible and one boundary-compressible. They then asked whether or not this is true in general. We utilize the results of this paper to answer that question in the negative with a specific example. In a recent paper (cf. [10]),Curtis and Taylor give a further corollary of Theorem 5.4, showing that the two checkerboard surfaces of a reduced alternating projection of an alternating knot must yield the maximum and minimum integral slopes for all essential boundary surfaces of the knot. This implies that these maximum and minimum integral slopes must be twice the number of positive crossings and the negative of twice the number of negative crossings in the projection respectively. Since all slopes are integral for Monesinos knots, this yields the maximum and minimum slopes for all surfaces in that case. A CLASSIFICATION OF SPANNING SURFACES FOR ALTERNATING LINKS 3 The layered surfaces that appear here have independently been considered previously in [27] (see footnote on p.10 of the ArXiv version) and in [26], where they are called state surfaces and where a criterion is provided that proves certain such surfaces are essential. 2. Background Standard definitions of reduced projections, flypes, positive and negative crossings, oriented links, incompressible and boundary-incompressible surfaces apply. A surface that is incompressible and boundary-incompressible is said to be essential. If a projection P contains p positive crossings and n negative crossings, then the writhe w(P ) is p − n. Writhe varies across projections of a given link and across orientations of a given projection, but is invariant across all reduced, alternating projections of an oriented link. We define the aggregate linking number lk(L) to be the sum of the linking numbers of all pairs of link components. We define the aggregate linking number of a knot to be zero. Equivalently, the aggregate linking number can be calculated in the same way as writhe, but only counting crossings between distinct link components, and halving at the end. Linking number and aggregate linking number are invariant across all projections of a given oriented link, but may well vary across various orientations of a given link. A projection divides the projection surface S2 into regions, each of whose boundary consists alter- nately of strands and crossings from L. A region with n crossings on its boundary will be called a projection n-gon. A spanning surface for a link L is a surface S ⊂ S3 with boundary equal to L such that S \@N(L) consists of one (p; 1)-curve on each component of @N(L), where p refers to the number of meridians. We will not distinguish between the spanning surface S as a subset of S3 and the spanning surface as a subset of S3nN(L), calling both S. Two spanning surfaces for L are considered equivalent if one is ambient isotopic to the other in S3nN(L). We will assume that S contains no closed components, so that every point in S is connected to L by a path in S. We need a special form of boundary-incompressibility for spanning surfaces. A spanning surface S in S3nN(L) is meridianally boundary-compressible if there exists a disk D embedded in S3nN(L) such that @D = α [ β where D \ S = β and D \ @N(L) = α are both arcs, β does not cut a disk off S, @D \ @S cuts @S into two arcs φ1 and φ2, and α [ φi is a meridian of the knot for at least one of i = 1; 2.
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