# 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 KL 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:
$\lambda(\Sigma(p,q,r))=-\frac{1}{8}\left[1-\frac{1}{3pqr}\left(1-p^2q^2r^2+p^2q^2+q^2r^2+p^2r^2\right) -d(p,qr)-d(q,pr)-d(r,pq)\right]$
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 λCW 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(MK, 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 MK, 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 λCWL of the Casson-Walker invariant to oriented compact 3-manifolds. It is uniquely characterized by the following properties:

$\lambda_{CWL}(M)=\tfrac{1}{2}\left\vert H_1(M)\right\vert\lambda_{CW}(M)$.
• If the first Betti number of M is one,
$\lambda_{CWL}(M)=\frac{\Delta^{\prime\prime}_M(1)}{2}-\frac{\mathrm{torsion}(H_1(M,\mathbb{Z}))}{12}$
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)

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
• 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