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Questions about the branch of combinatorics called graph theory (not to be used for questions concerning the graph of a function). This tag can be further specialized via using it in combination with more specialized tags such as extremal-graph-theory, spectral-graph-theory, algebraic-graph-theory, topological-graph-theory, random-graphs, graph-colorings and several others.

7 votes
Accepted

On the maximum number of $t$-subset of $\{1,\ldots, n\}$ having pairwise singleton or empty ...

Lucia's answer given in the linked question from the comment gives an upper bound (i.e., no pair should appear twice as a subset of $A_i$). But of course, the real question starts from here: When can …
Yuichiro Fujiwara's user avatar
3 votes
Accepted

chromatic number of a simple graph whose length of the longest odd cycle is 2k+1

Yes. If a graph $G$ is bipartite, by definition its chromatic number $\chi(G)$ is less than or equal to $2$. If $G$ contains an odd cycle, then $\chi(G) \leq l+1$, where $l$ is the length of a longest …
Yuichiro Fujiwara's user avatar