Let $\widehat{G}$ be the graph obtained by adding a vertex to a graph $G$ and joining it to all vertices in $V(G)$. Let $\sigma(G)$ be the number of non-positive eigenvalues of the adjacency matrix of $G$. By the interlacing theorem we have that $$\sigma(\widehat{G}) \in \{ \sigma(G), \sigma(G)+1\}\,.$$


Let us say that $G$ is *inertia friendly* whenever $\sigma(\widehat{G}) = \sigma(G)+1$. Making a simple computation it is not hard to see that any $r$-regular graph $(r > 0)$ is inertia friendly and so are all connected graphs on up to $4$ vertices.


The only two connected graphs of order $5$ that are not inertia-friendly are the $5$-path $P\_5$ and the [bull graph](https://en.wikipedia.org/wiki/Bull_graph). Computing the sequence of connected graphs that are not inertia friendly one obtains the numbers $0,0,0,0,2,10,131,1580,42228,\ldots$


Given these lists its easy to verify that all the graphs have $P\_4$ as an induced subgraph. Hence I wonder



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> Is it obvious that every [cograph](https://en.wikipedia.org/wiki/Cograph) is inertia friendly?
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It seems that a high proportion of connected graphs is inertia-friendly hence I wonder whether its possible to establish any other non-trivial properties about such graphs



> 
> What are some structural properties of inertia-friendly graphs?
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I would like to understand inertia-friendly graphs so any property that distinguishes them is useful. 


**Note.** The characteristic polynomial of $\widehat{G}$ is given by $$p\_{\widehat{G}}(x) = (x+1)p\_{G}(x) + (-1)^{n+1} p\_{\overline{G}}(-x-1)\,,$$ where $n$ is the order of $G$.



Inertia of the cone graph
Given a cograph $G$ it can be shown by induction (on the order of $G$) that $\widehat{G}$ (as defined above) has exactly one more negative eigenvalue than $G$. 


Useful facts:


* All cographs have a pair of vertices with either the same open or closed neighborhoods, respectively duplicate and coduplicate vertices.
* No cograph has an eigenvalue in the range (-1,0).
* Deletion of a duplicate vertex decreases the multiplicity of 0 in the resulting graph by 1.
* Deletion of a coduplicate vertex decreases the multiplicity of -1 in the resulting graph by 1.
* The rank of a cograph is equal to the number of distinct non zero rows.
* Interlacing


A corollary is that all cographs without isolated vertices are inertia friendly.


What was your motivation for considering inertia-friendly graphs?


