This is a question I found on exercise book. But I dont think it is true.
For example, this graph:
It is an upsided triangle on the top, and a triangle at the bottom. In this case, $\lambda(G) > r/2$.
This is a question I found on exercise book. But I dont think it is true.
For example, this graph:
It is an upsided triangle on the top, and a triangle at the bottom. In this case, $\lambda(G) > r/2$.
That graph isn't regular. The middle vertex has $4$ neighbors, but the other vertices have $2$.