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arXiv 1104.4018math.GT
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链生成的Alexander Quandle下界

Alexander quandle lower bounds for link genera

Riccardo Benedetti, Roberto Frigerio

摘要:

用Q_F表示有限域支持的Alexander quandle结构族.对于每个k分量定向链路L,L的每个划分P到h:= |P|子链路,以及这样的划分的每个由自然数z_1,.,z_n,(L,z)的任意图的X-染色数是(L,P)的一个定义良好的不变量,对于某个自然数a_X(L,P,z),其形式为q^(a_X(L,P,z)+1)。设X和z在Q_F中变化,在P的标号中,我们定义了一个导出不变量A_Q(L,P)= supa_X(L,P,z).如果P_M使得|P_M| =k,则我们证明A_Q(L,P_M)是t(L)的下界,其中t(L)是L的隧道数。若P是L的一个“边界剖分”,g(L,P)表示L_j的不相交Seifert曲面系的亏格之和的下确界,则A_Q(L,P)至多为2g(L,P)+2k- |P|-1。我们设置A_Q(L):=A_Q(L,P_m),其中|P_m| =1。通过对Inoue的一个结果的适当的改进,我们证明了当L=K是纽结时,A_Q(K)上有界于A(K),其中A(K)是K的Alexander多项式的宽度.然而,对于每个g,我们展示了具有相同Alexander多项式但不同quandle不变量A_Q的genus-g结的例子。此外,在这样的例子中,A_Q为纽结的属提供了尖锐的下界。另一方面,当L至少有两个分支时,A_Q(L)能给出比A(L)更好的亏格下界.我们证明了为了计算A_Q(L),只考虑关于常数标号z=1的着色就足够了。当L=K是纽结时,若A_Q(K)=A(K)或A_Q(K)为纽结亏格提供了一个尖锐的下界,或A_Q(K)=1,则A_Q(K)可由真量子族X=(F_p,*)实现,其中p在奇素数中变化.

英文摘要:

We denote by Q_F the family of the Alexander quandle structures supported by finite fields. For every k-component oriented link L, every partition P of L into h:=|P| sublinks, and every labelling z of such a partition by the natural numbers z_1,...,z_n, the number of X-colorings of any diagram of (L,z) is a well-defined invariant of (L,P), of the form q^(a_X(L,P,z)+1) for some natural number a_X(L,P,z). Letting X and z vary in Q_F and among the labellings of P, we define a derived invariant A_Q(L,P)=sup a_X(L,P,z). If P_M is such that |P_M|=k, we show that A_Q(L,P_M) is a lower bound for t(L), where t(L) is the tunnel number of L. If P is a "boundary partition" of L and g(L,P) denotes the infimum among the sums of the genera of a system of disjoint Seifert surfaces for the L_j's, then we show that A_Q(L,P) is at most 2g(L,P)+2k-|P|-1. We set A_Q(L):=A_Q(L,P_m), where |P_m|=1. By elaborating on a suitable version of a result by Inoue, we show that when L=K is a knot then A_Q(K) is bounded above by A(K), where A(K) is the breadth of the Alexander polynomial of K. However, for every g we exhibit examples of genus-g knots having the same Alexander polynomial but different quandle invariants A_Q. Moreover, in such examples A_Q provides sharp lower bounds for the genera of the knots. On the other hand, A_Q(L) can give better lower bounds on the genus than A(L), when L has at least two components. We show that in order to compute A_Q(L) it is enough to consider only colorings with respect to the constant labelling z=1. In the case when L=K is a knot, if either A_Q(K)=A(K) or A_Q(K) provides a sharp lower bound for the knot genus, or if A_Q(K)=1, then A_Q(K) can be realized by means of the proper subfamily of quandles X=(F_p,*), where p varies among the odd prime numbers.

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