homomorphism density

Let $G=(V,E)$ and $F=(V^{\prime },E^{\prime })$ be graphs. The homomorphism density from $F$ to $G$, denoted $t(F,G)$ is given as

Where $\text{hom}(F,G)$ is the the total number of homomorphisms from $F$ to $G$.

We can think of this as the probability of sampling a version of $F$ from the graph $G$.
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