mxty42ued

2022-11-17

Maximum number of edges in a graph satisfying conditions
There is a graph with 40 vertices. It is known that any edge has at least one endpoint, on which no more than four other edges are incident. What is the maximum number of edges that this graph can have? No multiple edges or loops are allowed.

Nigerkamg5

Expert

Step 1
For a vertex v with a neighbours of degree $\le 5$ and b neihgbours of degree $>5$ we define f(v) as $a/2+b$.
Notice $m=\sum _{v|d\left(v\right)\le 5}^{n}f\left(v\right)$.
Step 2
Let k be the number of vertices of degree $\le 5$.
If $k\le 35$ then clearly $m\le 35\cdot 5=175$.
If $k=35+l$ with $k\in \left\{1,\dots ,5\right\}$ then each summand is at most $5-\frac{l}{2}$ so $\sum _{v|d\left(v\right)\le 5}^{n}f\left(v\right)\le \left(35+l\right)\left(5-\frac{l}{2}\right)<175$.

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