I am looking for a large family (infinite pairs) of cospectral graphs with these condtions:

  • The graphs are non-regular,

  • Minimum degree is greater than $1$,

  • The degree sequences of these cospectral graphs are the same.

I need cospectrality by adjacency matrix and the graphs are simple.

The motivation for asking this question is that; if we have counterexample for reconstruction conjecture, the two graphs have these properties. I want to study such family of graphs.


Let $D$ be a Steiner triple system on $v$ points. (So $v\equiv1,3$ mod 6). The incidence graph is the bipartite graph with the $v$ points as one colour class and the $v(v-1)/6$ blocks as the second; a point is incident with the $(v-1)/2$ blocks that contain it.

Let $N$ be the point-block incidence matrix of the system. Then $NN^T=\frac12(v-3)I+J$ and $NN^T$ has the same non-zero eigenvalues as $NN^T$, with the same multiplicities. The adjacency matrix $A$ has the form $$A =\begin{pmatrix}0&N\\ N&0\end{pmatrix}$$ and $$A^2 =\begin{pmatrix}NN^T&0\\ 0&N^TN\end{pmatrix}$$ from which it follows that spectrum of $A$ is determined by $v$.

The incidence graphs have degree set $\{3,(v-1)/2\}$, so these graphs are not regular if $v\ge9$. There are 80 Steiner triple systems on 15 points and Kaski and Ostergard showed that there are 11,084,874,829 on 19 points.

The number of walks of length $k$ in an incidence graph is determined by $v$ (exercise) and it follows that the complements of the incidence graphs are also cospectral.

  • $\begingroup$ Dear Godsil, thanks for your good answer. Do you know other family of such graphs that has more diversity on degree sequences? $\endgroup$ – Shah Rooz Mar 21 at 20:31
  • 1
    $\begingroup$ @Shah Rooz: for a start, Cartesian products of the above. $\endgroup$ – Chris Godsil Mar 21 at 23:27

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