# Derivative of Tutte polynomial at -1

Let Tutte polynomial on graph with edge-set $E$ be defined as follows

$$f(q,v)=\sum_{A\subseteq E} q^{\kappa(A)} v^{|A|}$$

Here the sum is over all subgraphs $A$, $\kappa(A)$ is the number of connected components in $A$, $|A|$ is number of edges in $A$.

Let $$g(q,v)=\frac{\partial}{\partial v} \log f(q,v)$$ $$h(q,v)=\frac{\partial}{\partial v} f(q,v)$$

Is anything known about $g(q,-1)$ or $h(q,-1)$?

(Edit 10/26: actually it's the multivariate Tutte polynomial restricted to all $v$'s being equal, relation to regular Tutte polynomial in Jeremy Martin's answer)

Update 10/24

$f(q,-1)$ is the number of proper $q$ colorings, $h(q,-1)$ is ???, $g(q,-1)$ is their ratio. Below are tables of values of $h(q,-1)$ for paths, cycles, complete graphs. Columns give graph size, $n=3..6$, rows give $q=2..8$

Mathematica code to generate this

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## 2 Answers

This polynomial isn't the usual Tutte polynomial, but it's equivalent. Provided that $G$ is connected (which is probably the case you're interested in, and I'll assume), it looks like $f_G(q,v)=qv^{n-1}T_G(q/v+1,v+1)$, where $n$ is the number of vertices of $G$.

One thing that comes to mind is the Crapo beta invariant, which is $(-1)^n\chi_G'(1)$, where $\chi_G(k)=(-1)^{n-1}kT_G(1-k,0)$ is the chromatic polynomial. (E.g., see exercise 22 of lecture 4 of Stanley's notes on hyperplane arrangements: http://www-math.mit.edu/~rstan/arrangements/arr.html.) Your polynomials, especially $g$, might be related to this, but I haven't thought about the details.

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The polynomial as given is actually just the partition function of the q-state Potts model; this is known to be equivalent to the Tutte Polynomial, but often it is useful to work with it in the given form. – Gordon Royle Oct 26 '10 at 23:56

But if anyone is interested in formal derivatives of the original Tutte polynomial (for Matroids), there is a nice expression of its derivatives in terms of activities (which of course is how Tutte defined his polynomial originally) in this paper of Las Vergnas: http://arxiv.org/pdf/1205.5247.pdf.

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