No triangles or rectangles in a Moore graph of diameter 2.

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Can somebody explain why there cannot be any triangles or squares in a Moore graph with diameter 2? This was stated without proof in my class.

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I don't know which definition you're using, but on Wikipedia, a useful one is that a Moore graph is a graph $G$ of diameter $k$ and girth $2k + 1$.

That means if $k = 2$, then the smallest cycle in $G$ has $5$ vertices. Having a triangle or a square would contradict that.