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We know that for a simple graph (i.e. a graph with no self-loops and multiedges), the Graph Laplacian uniquely characterizes it in the sense that if two graphs have the same Graph Laplacian, then the graphs are the same. I was wondering whether a similar property holds for Hodge Laplacians.

By this I mean, if two simplicial complexes have the same sequence of Hodge Laplacians (also known as p-Laplacian if it calculated for an element belonging to chain group of p-simplices), then they must be the same simplicial complex (upto reordering of the vertices or rearrangement of the orientations of the subsimplices in the simplical complex). If the above statement is false, can a counterexample be constructed? While providing counterexamples, it will be appreciated, but not necessary, if the intuition used for constructing it is described.

For relevant definitions, I would request all to refer to http://pi.math.cornell.edu/~goldberg/Papers/CombinatorialLaplacians.pdf.

I am very new to this field, so I apologize if the answer to this question is very obvious.

It would be very helpful if a diagram is also provided while referring to any particular topological space (such as a torus) or simplicial complex.

EDIT: Following Connor Malin's suggestion, I am stating the definitions used.

For each boundary operator $\partial_{d}: C_{d} \rightarrow C_{d-1}$ of $K,$ we let $\mathcal{B}_{d}$ be the matrix representation of this operator relative to the standard bases for $C_{d}$ and $C_{d-1}$ with some orderings given to them. Let $\partial_{d}^{*}$ be the coboundary operator, which for our purpose is the linear adjoint of the boundary operator of $\partial_{d}$. So matrix representation of $\partial_{d}^{*}$ relative to the standard bases for $C_{d}$ and $C_{d-1}$ using the same orderings as for $\partial_{d}$ is $\mathcal{B}_{d}^{T}$.

Let $K$ be a finite oriented simplicial complex, and let $d \geq 0$ be an integer. The $d$ th combinatorial Laplacian is the linear operator $\Delta_{d}: C_{d} \rightarrow C_{d}$ given by $$ \Delta_{d}=\partial_{d+1} \circ \partial_{d+1}^{*}+\partial_{d}^{*} \circ \partial_{d} $$ For convenience, we use the notations $\Delta_{d}^{U P}=\partial_{d+1} \circ \partial_{d+1}^{*}$ and $\Delta_{d}^{D N}=\partial_{d}^{*} \circ \partial_{d},$ so that $\Delta_{d}=\Delta_{d}^{U P}+\Delta_{d}^{D N}$

The $d$ th Laplacian matrix (Hodge Laplacian) of $K$, denoted $\mathcal{L}_{d}$, relative to some orderings of the standard bases for $C_{d}$ and $C_{d-1}$ of $K,$ is the matrix representation of $\Delta_{d}$. Observe that $$ \mathcal{L}_{d}=\mathcal{B}_{d+1} \mathcal{B}_{d+1}^{T}+\mathcal{B}_{d}^{T} \mathcal{B}_{d} $$ As above, for convenience, we use the notations $\mathcal{L}_{d}^{U P}=\mathcal{B}_{d+1} \mathcal{B}_{d+1}^{T}$ and $\mathcal{L}_{d}^{D N}=\mathcal{B}_{d}^{T} \mathcal{B}_{d},$ so that $\mathcal{L}_{d}=\mathcal{L}_{d}^{U P}+\mathcal{L}_{d}^{D N}$

Note: $d = 0$ refers to the (unnormalized) graph Laplacian.

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A counter example is given by the torus and the wedge of a sphere and two circles. The attaching maps from the 1-simplices to the 0-simplices are the same in both cases, so the 0th Hodge Laplacian is the same.

The 1st Hodge Laplacian is the same because the attaching map of the 1-simplices to the 0-simplices are the same, again, plus the boundary map from the 2-chains to the 1-chains is 0 in both cases, so the only contribution comes from the boundary map between the 1 and 0 simplices.

The 3rd Hodge Laplacian is the same in both cases because there are no higher simplices and the boundary map from the 2-chains to the 1-chains in both cases are 0.

Maybe an intuitive reason why the 1-dimensional case is different is that 1-dimensional spaces have no interesting cup products, but higher dimensions do. When we use the chain complex to define an invariant, we most likely have no interaction with the cup product which comes from the cochains. Good to note is that these spaces I gave have the same homology and cohomology, but are distinguished by their cup product.

Connor Malin
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  • Please correct me if I am wrong, but I don't think a torus is a simplicial complex, neither is the wedge of a sphere and two circles. Are you trying to refer to some simplicial complex structure (like a Vietoris-Rips complex constructed from points sampled on the torus and the wedge of a sphere and two circles, respectively) and computing the Hodge Laplacians of those complexes? – Soumya Mukherjee Jul 27 '20 at 21:57
  • These are definitely both simplicial complexes, I realize I calculated with the analogous definition for CW complexes. There are many more simplices in the simplicial complex structure of these spaces. I'll leave my answer up because it does dismiss a more general version of the question, but I realize now it does not answer your question as stated. You might try to figure out how these are simplicial complexes (Hatcher for example probably has the Torus picture) and see if it still gives a counter example. – Connor Malin Jul 27 '20 at 22:22
  • Pardon me again, but could you provide a reference to the object "wedge of a sphere and two circles"? It is something new to me. – Soumya Mukherjee Jul 28 '20 at 05:09
  • @SoumyaMukherjee The wedge of two topological objects with distinguished points is the topological object which is those two objects glued at the distinguished points. For simplicial complexes, we pick a vertex on each simplicial complex and take a disjoint union of the simplices of each complex with the exception that we identify the two vertices into one. – Connor Malin Jul 28 '20 at 13:59
  • @ConnorMalin it seems to me that the Laplacians coincide mainly because the CW complexes structures you chose are not regular, i.e., a lot of cancellation appear at boundaries. I'm just curious what if we are restricted to regular CW complexes. Do you have any hunch? – Chaos Mar 12 '21 at 22:19
  • @Chaos I am not sure. I have not really ever messed with regular CW complexes; I read that they are all triangulable, so you might try to look at the proof of that and try to see what happens with these Laplacian operators. – Connor Malin Mar 13 '21 at 15:55