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Bordered Floer homology and existence of incompressible tori in homology spheres

Published online by Cambridge University Press:  18 May 2018

Eaman Eftekhary*
Affiliation:
School of Mathematics, Institute for Research in Fundamental Sciences (IPM), P. O. Box 19395-5746, Tehran, Iran email eaman@ipm.ir
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Abstract

Let $Y$ be a homology sphere which contains an incompressible torus. We show that $Y$ cannot be an $L$ -space, i.e. the rank of $\widehat{\text{HF}}(Y)$ is greater than $1$ . In fact, if the homology sphere $Y$ is an irreducible $L$ -space, then $Y$ is $S^{3}$ , the Poincaré sphere $\unicode[STIX]{x1D6F4}(2,3,5)$ or hyperbolic.

Information

Type
Research Article
Copyright
© The Author 2018 
Figure 0

Figure 1. The winding region and the arrangement of the curves $\unicode[STIX]{x1D706}_{n},\unicode[STIX]{x1D706}_{n+m},\unicode[STIX]{x1D706}_{\infty },\unicode[STIX]{x1D706}_{\infty }^{\prime }$ and $\unicode[STIX]{x1D6FC}_{g}$ in this region, as well as the markings $a,b,t,u,v$ and $z$, are illustrated. The winding region is obtained after we identify the upper edge of the rectangle with its lower edge using a reflection. The intersection points of $\unicode[STIX]{x1D6FC}_{g}$ with each one of the curves $\unicode[STIX]{x1D706}_{n},\unicode[STIX]{x1D706}_{n+m},\unicode[STIX]{x1D706}_{\infty },\unicode[STIX]{x1D706}_{\infty }^{\prime }$ is labelled. A triangle with vertices $p_{n},q$ and $p_{n+m}$ containing the marked point $a$, and another triangle with vertices $q,x_{i}$ and $x_{i}^{n}$ missing the marked points $a,u$ and $v$, are shaded.

Figure 1

Figure 2. The curves $\unicode[STIX]{x1D706}_{\infty }^{\prime }$ and $\unicode[STIX]{x1D706}_{n}^{\prime }$ are Hamiltonian isotopes of the curves $\unicode[STIX]{x1D706}_{\infty }$ and $\unicode[STIX]{x1D706}_{n}$, respectively. The triangle $\unicode[STIX]{x1D6E5}_{a}$ is decomposed to two small triangles $[p_{n}q_{n}p_{n}^{\prime }]$ and $[q_{\infty }p_{n+m}^{\prime }p_{n+m}]$ and a pentagon. The union of the pentagon with the first triangle is a rectangle with vertices $q,p_{n+m},p_{n}^{\prime }$ and $q_{n}$, which is lightly shaded, and its union with the second triangle is a rectangle with vertices $p_{n},q,p_{n+m}^{\prime },q_{\infty }$, which is strongly shaded.

Figure 2

Figure 3. The arrangement of the curves in the winding region. The upper edge and the lower edge of the rectangle are identified by a reflection to form the winding region. The shaded area on the right is the intersection of the domain of a square class $\Box _{0}$, which is in $\unicode[STIX]{x1D70B}_{2}^{0}(\mathbf{w}_{0}^{(0)},\unicode[STIX]{x1D6E9}_{0,1},\unicode[STIX]{x1D6E9}_{g_{1}},\mathbf{w}_{0};u,v,w,b)$, with the winding region. It is a $4$-gon with vertices $x_{0}^{0},q_{0},q_{m}$ and $x_{0}$. The shaded area on the left is the intersection of the domain of a triangle class $\unicode[STIX]{x1D6E5}_{0}\in \unicode[STIX]{x1D70B}_{2}^{0}(\mathbf{w}_{1}^{(0)},\unicode[STIX]{x1D6E9}_{g_{0}}^{\prime },\mathbf{w}_{1};u,v,w)$ with the winding region, which is a triangle with vertices $x_{1}^{0},q^{\prime }$ and $x_{1}$.

Figure 3

Figure 4. In the Heegaard diagram $(\unicode[STIX]{x1D6F4},\unicode[STIX]{x1D736},\unicode[STIX]{x1D737},\unicode[STIX]{x1D737}_{0},\unicode[STIX]{x1D737}_{m};u,v,w)$, we may take out the shaded area from the winding region and glue back the boundaries after shifting the left-hand-side part to the right to obtain a Heegaard diagram which may be identified with $(\unicode[STIX]{x1D6F4}^{\prime },\unicode[STIX]{x1D736},\unicode[STIX]{x1D737},\unicode[STIX]{x1D737}_{0},\unicode[STIX]{x1D737}_{m-1};u,v)$, where $\unicode[STIX]{x1D6F4}^{\prime }$ is the surface $\unicode[STIX]{x1D6F4}$ but with a different complex structure.

Figure 4

Figure 5. A domain, which is an immersion of a triangle on the surface, is lightly shaded. This domain has vertices $q_{0},q_{m}$ and $q^{\prime }$. The coefficient of the shaded domain in the small triangle with vertices $q_{0},q_{m}$ and $q$ (which is strongly shaded) is $2$, while its coefficient in the rest of the shaded areas is $1$. Moreover, a 4-gon with vertices $q_{m},p_{m},p_{\infty }$ and $p_{1}^{\prime }$ is strongly shaded.