Asymmetric Lovász local lemma 2026-10-06
For a finite family with a dependency graph of events, if satisfy , then . Choosing equal gives the familiar symmetric criterion when event probabilities are at most and the maximum dependency degree is .
Dependency graph of events 2026-10-06
A dependency graph of events joins potentially dependent events. Each event must be independent of the sigma-algebra generated by all its nonneighbors, a joint condition stronger than pairwise independence. In a product probability space, joining events whose sets of underlying independent variables overlap gives such a graph. This supplies the hypothesis of the Lovász local lemma.
Past exam of the mathematics course of the University of Cambridge 2014 iii Paper 11 4 ii Solution Created 2026-10-03 Updated 2026-10-06
Color each integer independently and uniformly with one of colors. For a translate , let be the event that some color is missing. The union bound givesJoin two events in the dependency graph of events when their translates overlap. Nonneighbors depend on disjoint sets of independent colors, giving the required joint independence. Overlap occurs only if , so the maximum degree is at most .
With natural logarithms, and implyThe middle estimate uses that is decreasing for , and the next uses . Therefore the Lovász local lemma supplies a coloring avoiding every bad event in any specified finite family of translates.
To obtain one coloring for all translates, use the compactness extension of the Lovász local lemma. At level , consider colorings of satisfying every constraint whose translate is contained in that interval. There are finitely many constraints, and the preceding argument makes the level nonempty. Restrictions connect these colorings into a finitely branching tree. The König infinity lemma gives an infinite branch, hence a coloring of all integers. Every translate eventually lies in a level of this branch, so it contains every color.
Thus there exists a -coloring of in which every translate of contains all colors. This is a polychromatic coloring of integer translates; the compactness step establishes existence of one simultaneous coloring.
Past exam of the mathematics course of the University of Cambridge 2014 iii Paper 11 4 i Solution Created 2026-10-03 Updated 2026-10-06
An independence graph of events, also called a dependency graph of events, has one vertex for each event , with independent of the entire family of events indexed by its nonneighbors. Precisely, is independent of the sigma-algebra generated by those events. Pairwise independence alone is insufficient.
The asymmetric Lovász local lemma says that if numbers satisfythen for a finite family. In the commonly used symmetric Lovász local lemma, if each probability is at most and the maximum graph degree is at most , the sufficient condition is
Polychromatic coloring of integer translates 2026-10-06
For finite , use vertices and hyperedges . Independent uniform -coloring gives missing-color probability at most , and the dependency graph of events has degree at most . For and , the symmetric Lovász local lemma condition holds. The compactness extension of the Lovász local lemma gives a coloring in which every translate contains every color.