- 2-SAT is in P; 3-SAT is NP-complete.
- Exact cover by 2-sets is in P; exact cover by 3-sets is NP-complete
- Two-dimensional matching is in P; three-dimensional matching is NP-complete
- Graph 2-coloring is trivially in P; graph 3-coloring is NP-complete
Are there other well-known or important examples that are similar, in that there is a family of problems with a natural parameter $ n$ , where the problem is known to be in P when $ n<3$ and known to be NP-hard when $ n≥3$ ?
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