# Proof of $\det\partial_2^t∂_2 =m^2 ·k(G)$ for G, finite connected graph with reduced homology being 0

Let $G$ be a finite connected graph. Let $K$ be a 2-dimensional complex such that $K^{(1)} = G$, $\tilde{H}_2(K)=0$ and $\tilde{H}_1(K)=\Bbb Z_m$. Show that $\det\partial_2^t∂_2 =m^2 ·k(G).$

Over the above statement, the $\tilde{H}$ notates the reduced simplicial homology and $k(G)$ notates the number of spanning trees of given graph G.

I can't figure out where to start to prove the square of boundary operator results to be same as $m^2\cdot k(G)$?

This follows from Remark 4.17 / Proposition 4.10 of "Cuts and flows of cell complexes" by Duval, Klivans and Martin. Their formula (as applied to your problem, by taking $\Sigma=\Upsilon=K$) states:

$$\det(\partial^t_2\partial_2)=\sum_\Gamma |\mathbf{T}(\tilde H_{1}(K,\Gamma))|^2.$$

Here $\mathbf{T}(\cdot)$ denotes the torsion subgroup and the sum over $\Gamma$ runs over all "relatively acyclic" subcomplexes of $K$ (Definition 3.1). Since $\tilde{H}_1(K)$ is torsion, the discussion before Proposition 3.2 shows that the $\Gamma$ are in fact the spanning trees of $G$.

Furthermore, the relative homology groups $\tilde{H}_1(K,\Gamma)$ are all isomorphic to $\tilde{H}_1(K)=\mathbb{Z}_m$ because all the $\Gamma$'s are trees and hence contractible. The result is that the sum evaluates to $m^2 k(G)$ as desired.

Here follow a few general comments. The above formula is an interesting higher-dimensional variant of the Kirchhoff Matrix-Tree theorem which expresses the number of spanning trees of a graph in terms of the determinant of the "up-down" Laplacian $L^{ud}=\partial\partial^t$ (with a row and vertex removed). Contrast this with the "down-up Laplacian" $L^{du}=\partial_i^t\partial_i$ that's considered here. These two matrices are closely related (physicists might call them "supersymmetric partners") since the multisets of their nonzero eigenvalues are identical. However, it seems that $L^{ud}$ shows up more frequently in practice and hence is much more studied. I could be showing my ignorance but other than the above formula, I haven't seen much else on $L^{du}$ in the literature.

The "up-down / down-up" terminology can be found in Duval, Klivans and Martin's survey on simplicial and cellular trees which includes several higher-dimensional generalizations of Kirchhoff (all involving $L^{ud}$ though!).

• from the lecture note of the class of algebraic topology recommended to handle with. – Beverlie Jun 2 '18 at 7:02
• In your link there's no proposition 4.11 only 4.1 exist. – Beverlie Jun 2 '18 at 7:04
• Sorry, Prop. 4.11 was a typo for Prop. 4.10, as cited in Remark 4.17. This is quite a nice problem: which lecture notes by which professor? – j.c. Jun 2 '18 at 10:34
• why is it so good? I even don't reach the level to check how good it is. There's another pb set from the professor: math.stackexchange.com/questions/2804986/… – Beverlie Jun 2 '18 at 10:54
• chat.stackexchange.com/rooms/78346/algebraic-topology will you come in here? – Beverlie Jun 2 '18 at 11:06