In mathematics, the connective constant is a numerical quantity associated with self-avoiding walks on a lattice. It is studied in connection with the notion of universality in two-dimensional statistical physics models. While the connective constant depends on the choice of lattice so itself is not universal (similarly to other lattice-dependent quantities such as the critical probability threshold for percolation), it is nonetheless an important quantity that appears in conjectures for universal laws. Furthermore, the mathematical techniques used to understand the connective constant, for example in the recent rigorous proof by Duminil-Copin and Smirnov that the connective constant of the hexagonal lattice has the precise value , may provide clues to a possible approach for attacking other important open problems in the study of self-avoiding walks, notably the conjecture that self-avoiding walks converge in the scaling limit to the Schramm–Loewner evolution.
The connective constant is defined as follows. Let denote the number of n-step self-avoiding walks starting from a fixed origin point in the lattice. Since every n + m step self avoiding walk can be decomposed into an n-step self-avoiding walk and an m-step self-avoiding walk, it follows that . Then by applying Fekete's lemma to the logarithm of the above relation, the limit can be shown to exist. This number is called the connective constant, and clearly depends on the particular lattice chosen for the walk since does. The value of is precisely known only for two lattices, see below. For other lattices, has only been approximated numerically. It is conjectured that as n goes to infinity, where depends on the lattice, but the critical exponent is universal (it depends on dimension, but not the specific lattice). In 2-dimensions it is conjectured that 
These values are taken from the 1998 Jensen–Guttmann paper. The connective constant of the lattice, since each step on the hexagonal lattice corresponds to either two or three steps in it, can be expressed exactly as a root of the polynomial
given the exact expression for the hexagonal lattice connective constant. More information about these lattices can be found in the percolation threshold article.
In 2010, Hugo Duminil-Copin and Stanislav Smirnov published the first rigorous proof of the fact that for the hexagonal lattice. This had been conjectured by Nienhuis in 1982 as part of a larger study of O(n) models using renormalization techniques. The rigorous proof of this fact came from a program of applying tools from complex analysis to discrete probabilistic models that has also produced impressive results about the Ising model among others. The argument relies on the existence of a parafermionic observable that satisfies half of the discrete Cauchy–Riemann equations for the hexagonal lattice. We modify slightly the definition of a self-avoiding walk by having it start and end on mid-edges between vertices. Let H be the set of all mid-edges of the hexagonal lattice. For a self-avoiding walk between two mid-edges and , we define to be the number of vertices visited and its winding as the total rotation of the direction in radians when is traversed from to . The aim of the proof is to show that the partition function
converges for and diverges for where the critical parameter is given by . This immediately implies that .
Given a domain in the hexagonal lattice, a starting mid-edge , and two parameters and , we define the parafermionic observable
If and , then for any vertex in , we have
where are the mid-edges emanating from . This lemma establishes that the parafermionic observable is divergence-free. It has not been shown to be curl-free, but this would solve several open problems (see conjectures). The proof of this lemma is a clever computation that relies heavily on the geometry of the hexagonal lattice.
Next, we focus on a finite trapezoidal domain with 2L cells forming the left hand side, T cells across, and upper and lower sides at an angle of . (Picture needed.) We embed the hexagonal lattice in the complex plane so that the edge lengths are 1 and the mid-edge in the center of the left hand side is positioned at −1/2. Then the vertices in are given by
We now define partition functions for self-avoiding walks starting at and ending on different parts of the boundary. Let denote the left hand boundary, the right hand boundary, the upper boundary, and the lower boundary. Let
By summing the identity
over all vertices in and noting that the winding is fixed depending on which part of the boundary the path terminates at, we can arrive at the relation
after another clever computation. Letting , we get a strip domain and partition functions
It was later shown that , but we do not need this for the proof. We are left with the relation
From here, we can derive the inequality
And arrive by induction at a strictly positive lower bound for . Since , we have established that .
For the reverse inequality, for an arbitrary self avoiding walk on the honeycomb lattice, we perform a canonical decomposition due to Hammersley and Welsh of the walk into bridges of widths and . Note that we can bound
which implies . Finally, it is possible to bound the partition function by the bridge partition functions
And so, we have that as desired.
Nienhuis argued in favor of Flory's prediction that the mean squared displacement of the self-avoiding random walk satisfies the scaling relation , with . The scaling exponent and the universal constant could be computed if the self-avoiding walk possesses a conformally invariant scaling limit, conjectured to be a Schramm–Loewner evolution with .
- Madras, N.; Slade, G. (1996). The Self-Avoiding Walk. Birkhäuser. ISBN 978-0-8176-3891-7.
- H. Duminil-Copin; S. Smirnov (2010). "The connective constant of the honeycomb lattice equals ".
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- N. Beaton; J. de Gier; A. J. Guttmann (2011). "The critical fugacity for surface adsorption of SAW on the honeycomb lattice is ". Communications in Mathematical Physics 326 (3): 727. arXiv:1109.0358. doi:10.1007/s00220-014-1896-1.
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