Casson invariant

In 3-dimensional topology, a part of the mathematical field of geometric topology, the Casson invariant is an integer-valued invariant of oriented integral homology 3-spheres, introduced by Andrew Casson.

Kevin Walker (1992) found an extension to rational homology 3-spheres, called the Casson–Walker invariant, and Christine Lescop (1995) extended the invariant to all closed oriented 3-manifolds.

Definition

A Casson invariant is a surjective map λ from oriented integral homology 3-spheres to Z satisfying the following properties:

• λ(S³) = 0.
• Let Σ be an integral homology 3-sphere. Then for any knot K and for any integer n, the difference

$\lambda \left(\Sigma +{\frac {1}{n+1}}\cdot K\right)-\lambda \left(\Sigma +{\frac {1}{n}}\cdot K\right)$

is independent of n. Here $\Sigma +{\frac {1}{m}}\cdot K$ denotes ${\frac {1}{m}}$ Dehn surgery on Σ by K.

• For any boundary link K ∪ L in Σ the following expression is zero:

$\lambda \left(\Sigma +{\frac {1}{m+1}}\cdot K+{\frac {1}{n+1}}\cdot L\right)-\lambda \left(\Sigma +{\frac {1}{m}}\cdot K+{\frac {1}{n+1}}\cdot L\right)-\lambda \left(\Sigma +{\frac {1}{m+1}}\cdot K+{\frac {1}{n}}\cdot L\right)+\lambda \left(\Sigma +{\frac {1}{m}}\cdot K+{\frac {1}{n}}\cdot L\right)$

The Casson invariant is unique (with respect to the above properties) up to an overall multiplicative constant.

Properties

• If K is the trefoil then

$\lambda \left(\Sigma +{\frac {1}{n+1}}\cdot K\right)-\lambda \left(\Sigma +{\frac {1}{n}}\cdot K\right)=\pm 1$.

• The Casson invariant is 1 (or −1) for the Poincaré homology sphere.
• The Casson invariant changes sign if the orientation of M is reversed.
• The Rokhlin invariant of M is equal to the Casson invariant mod 2.
• The Casson invariant is additive with respect to connected summing of homology 3-spheres.
• The Casson invariant is a sort of Euler characteristic for Floer homology.
• For any integer n

$\lambda \left(M+{\frac {1}{n+1}}\cdot K\right)-\lambda \left(M+{\frac {1}{n}}\cdot K\right)=\phi _{1}(K),$

where $\phi _{1}(K)$ is the coefficient of $z^2$ in the Alexander-Conway polynomial $\nabla _{K}(z)$, and is congruent (mod 2) to the Arf invariant of K.

• The Casson invariant is the degree 1 part of the LMO invariant.
• The Casson invariant for the Seifert manifold $\Sigma (p,q,r)$ is given by the formula:

$\lambda (\Sigma (p,q,r))=-{\frac {1}{8}}\left[1-{\frac {1}{3pqr}}\left(1-p^{2}q^{2}r^{2}+p^{2}q^{2}+q^{2}r^{2}+p^{2}r^{2}\right)-d(p,qr)-d(q,pr)-d(r,pq)\right]$

where

$d(a,b)=-{\frac {1}{a}}\sum _{{k=1}}^{{a-1}}\cot \left({\frac {\pi k}{a}}\right)\cot \left({\frac {\pi bk}{a}}\right)$

The Casson invariant as a count of representations

Informally speaking, the Casson invariant counts half the number of conjugacy classes of representations of the fundamental group of a homology 3-sphere M into the group SU(2). This can be made precise as follows.

The representation space of a compact oriented 3-manifold M is defined as ${\mathcal {R}}(M)=R^{{{\mathrm {irr}}}}(M)/SO(3)$ where $R^{{{\mathrm {irr}}}}(M)$ denotes the space of irreducible SU(2) representations of $\pi _{1}(M)$. For a Heegaard splitting $\Sigma =M_{1}\cup _{F}M_{2}$ of $M$, the Casson invariant equals ${\frac {(-1)^{g}}{2}}$ times the algebraic intersection of ${\mathcal {R}}(M_{1})$ with ${\mathcal {R}}(M_{2})$.

Generalizations

Rational homology 3-spheres

Kevin Walker found an extension of the Casson invariant to rational homology 3-spheres. A Casson-Walker invariant is a surjective map λ_CW from oriented rational homology 3-spheres to Q satisfying the following properties:

1. λ(S³) = 0.

2. For every 1-component Dehn surgery presentation (K, μ) of an oriented rational homology sphere M′ in an oriented rational homology sphere M:

$\lambda _{{CW}}(M^{\prime })=\lambda _{{CW}}(M)+{\frac {\langle m,\mu \rangle }{\langle m,\nu \rangle \langle \mu ,\nu \rangle }}\Delta _{{W}}^{{\prime \prime }}(M-K)(1)+\tau _{{W}}(m,\mu ;\nu )$

where:

• m is an oriented meridian of a knot K and μ is the characteristic curve of the surgery.
• ν is a generator the kernel of the natural map H₁(∂N(K), Z) → H₁(M−K, Z).
• $\langle \cdot ,\cdot \rangle$ is the intersection form on the tubular neighbourhood of the knot, N(K).
• Δ is the Alexander polynomial normalized so that the action of t corresponds to an action of the generator of $H_{1}(M-K)/{\text{Torsion}}$ in the infinite cyclic cover of M−K, and is symmetric and evaluates to 1 at 1.
• $\tau _{{W}}(m,\mu ;\nu )=-{\mathrm {sgn}}\langle y,m\rangle s(\langle x,m\rangle ,\langle y,m\rangle )+{\mathrm {sgn}}\langle y,\mu \rangle s(\langle x,\mu \rangle ,\langle y,\mu \rangle )+{\frac {(\delta ^{2}-1)\langle m,\mu \rangle }{12\langle m,\nu \rangle \langle \mu ,\nu \rangle }}$

where x, y are generators of H₁(∂N(K), Z) such that $\langle x,y\rangle =1$, v = δy for an integer δ and s(p, q) is the Dedekind sum.

Note that for integer homology spheres, the Walker's normalization is twice that of Casson's: $\lambda _{{CW}}(M)=2\lambda (M)$.

Compact oriented 3-manifolds

Christine Lescop defined an extension λ_CWL of the Casson-Walker invariant to oriented compact 3-manifolds. It is uniquely characterized by the following properties:

• If the first Betti number of M is zero,

$\lambda _{{CWL}}(M)={\tfrac {1}{2}}\left\vert H_{1}(M)\right\vert \lambda _{{CW}}(M)$.

• If the first Betti number of M is one,

$\lambda _{{CWL}}(M)={\frac {\Delta _{M}^{{\prime \prime }}(1)}{2}}-{\frac {{\mathrm {torsion}}(H_{1}(M,{\mathbb {Z}}))}{12}}$

where Δ is the Alexander polynomial normalized to be symmetric and take a positive value at 1.

• If the first Betti number of M is two,

$\lambda _{{CWL}}(M)=\left\vert {\mathrm {torsion}}(H_{1}(M))\right\vert {\mathrm {Link}}_{M}(\gamma ,\gamma ^{\prime })$

where γ is the oriented curve given by the intersection of two generators $S_{1},S_{2}$ of $H_{2}(M;{\mathbb {Z}})$ and $\gamma ^{\prime }$ is the parallel curve to γ induced by the trivialization of the tubular neighbourhood of γ determined by $S_{1},S_{2}$.

• If the first Betti number of M is three, then for a,b,c a basis for $H_{1}(M;{\mathbb {Z}})$, then

$\lambda _{{CWL}}(M)=\left\vert {\mathrm {torsion}}(H_{1}(M;{\mathbb {Z}}))\right\vert \left((a\cup b\cup c)([M])\right)^{2}$.

• If the first Betti number of M is greater than three, $\lambda _{{CWL}}(M)=0$.

The Casson–Walker–Lescop invariant has the following properties:

• If the orientation of M, then if the first Betti number of M is odd the Casson–Walker–Lescop invariant is unchanged, otherwise it changes sign.
• For connect-sums of manifolds

$\lambda _{{CWL}}(M_{1}\#M_{2})=\left\vert H_{1}(M_{2})\right\vert \lambda _{{CWL}}(M_{1})+\left\vert H_{1}(M_{1})\right\vert \lambda _{{CWL}}(M_{2})$

SU(N)

In 1990, C. Taubes showed that the SU(2) Casson invariant of a 3-homology sphere M has gauge theoretic interpretation as the Euler characteristic of ${\mathcal {A}}/{\mathcal {G}}$, where ${\mathcal {A}}$ is the space of SU(2) connections on M and ${\mathcal {G}}$ is the group of gauge transformations. He led Chern–Simons invariant as a $S^{1}$-valued Morse function on ${\mathcal {A}}/{\mathcal {G}}$ and pointed out that the SU(3) Casson invariant is important to make the invariants independent on perturbations. (Taubes (1990))

Boden and Herald (1998) defined an SU(3) Casson invariant.

References

• S. Akbulut and J. McCarthy, Casson's invariant for oriented homology 3-spheres— an exposition. Mathematical Notes, 36. Princeton University Press, Princeton, NJ, 1990. ISBN 0-691-08563-3
• M. Atiyah, New invariants of 3- and 4-dimensional manifolds. The mathematical heritage of Hermann Weyl (Durham, NC, 1987), 285–299, Proc. Sympos. Pure Math., 48, Amer. Math. Soc., Providence, RI, 1988.
• H. Boden and C. Herald, The SU(3) Casson invariant for integral homology 3-spheres. J. Differential Geom. 50 (1998), 147–206.
• C. Lescop, Global Surgery Formula for the Casson-Walker Invariant. 1995, ISBN 0-691-02132-5
• N. Saveliev, Lectures on the topology of 3-manifolds: An introduction to the Casson Invariant. de Gruyter, Berlin, 1999. ISBN 3-11-016271-7 ISBN 3-11-016272-5
• Taubes, Clifford Henry (1990), "Casson's invariant and gauge theory.", J. Differential Geom., 31: 547–599
• K. Walker, An extension of Casson's invariant. Annals of Mathematics Studies, 126. Princeton University Press, Princeton, NJ, 1992. ISBN 0-691-08766-0 ISBN 0-691-02532-0