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A graph is $d$ – regular if every vertex has degree $d$. For a $d$ – regular graph on $n$ vertices, which of the following must be TRUE?

1. $d$ divides $n$
2. Both $d$ and $n$ are even
3. Both $d$ and $n$ are odd
4. At least one of $d$ and $n$ is odd
5. At least one of $d$ and $n$ is even

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We know that the sum of degrees is twice the number of edges.

Now, sum of degrees is $nd$, as there are $n$ vertices of degree $d$ each.

As $nd$ is even, either one of them should be definitely even.
by Active (1.1k points)

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