What is the minimum number of matchings needed to cover all the edges of a graph?

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Given a graph, what is the minimum number of matchings needed such that their union contains all the edges in the graph? Is it related to the degree of the graph?

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In other words, you're asking about the minimum number of colors needed to color all the edges of a graph $G$ so that adjacent edges have different colors. That number is called the "edge chromatic number" or "chromatic index" of $G$, and is denoted by the symbol $\chi'(G)$.

By Vizing's theorem, $\Delta(G)\le\chi'(G)\le\Delta(G)+1$ where $\Delta(G)$ is the maximum degree of $G$. For example, $\chi'(K_n)=\Delta(K_n)=n-1$ for even $n$, while $\chi'(K_n)=\Delta(K_n)+1=n$ for odd $n\gt1$.