Casson invariant

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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.


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.


  • 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.
\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.
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[edit]

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).


Rational homology 3-spheres[edit]

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)


  • 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[edit]

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,
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)


In 1990, C. Taubes showed that the SU(2) Casson invarinat 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.


  • 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