Past exam of the mathematics course of the University of Cambridge 2007 iii Paper 13 2 Solution Created 2026-10-03 Updated 2026-10-14
The claimed sufficient condition is false for the printed all-colours requirement. This is a polychromatic hypergraph colouring problem, which is stronger than merely avoiding monochromatic edges.
For a counterexample even with , take and all four triples as edges. Every edge meets the other three, so , andAny assignment of three colours to four vertices repeats a colour on a pair. Any triple containing that pair has at most two colours. Thus this uniform hypergraph satisfies the numerical hypothesis but has no required colouring. A single two-vertex edge would already provide a smaller counterexample if is allowed.
Here is the correlation graph of events argument that correctly applies to the printed colouring definition. Colour each vertex independently and uniformly with three colours, and let be the event that edge misses some colour. For , the inclusion-exclusion principle givesJoin two bad events when their edges intersect. This is a dependency graph of events, hence also a correlation graph of events, of maximum degree at most : each depends only on colours on , which are jointly independent of the colours on all disjoint edges. Precisely, the symmetric Lovász local lemma says that events of probability at most with such a graph have positive simultaneous avoidance probability whenever . It follows from the asymmetric form , taking for and using ; the case is direct independence.
Thus a valid replacement sufficient condition isor the simpler, stronger condition . Under either replacement, avoiding all gives exactly the requested polychromatic hypergraph colouring. For infinitely many vertices, finite subfamilies can be coloured and compactness of the product of three-point spaces extends this to all constraints; positive simultaneous avoidance probability is only asserted for finite families. The printed bound cannot be justified by substituting monochromatic-event probabilities for missing-colour probabilities.