Monday, 6 June 2011

gn.general topology - covers of Zinfty

The answer is NO even if we replace 4 by 3.
Let me sketch a proof. This is based upon the following lemma.



Lemma. Fix S>0 and for an integer k conisder in mathbbZk sets X of diameter at most S. Denote by Vol(X) the number of points in X and denote by X1 the set of points of distance at most 1 from X. Now let delta(S,k) be the supremum over all X of diameter at most S of the ratio:



r(S,k)=supXsubsetmathbbZkfracVol(X)Vol(X1).



I claim that for a fixed S, limktoinftyr(S,k)=0.



Let us skip the proof of the lemma and instead deduce the claim. Suppose by contradiction that the answer is positive. Then for every k we will get a solution to the problem in mathbbZk with the fixed number of sets (U0,...,Un) such that each Uij is of the diameter at most S. Now, chose such k that r(S,k)<frac12n and let us deduce the contradiction.



From Lemma it follows that the supremum of asimptotic density of each set Ui in mathbbZk is less than frac1n+1. Indeed, since the distance between different components of Ui is 4, every point of Ui1 that does not belong to Ui is on distance one from at most one component of Ui. And lemma gives us the inequality (that should be understood as assymptotic in mathbbZk)
Vol(Ui)<frac12nVol(Ui1)lefrac12nVol(mathbbZk)


Hence mathbbZk can not be covered by U0,...,Un.



It is clear where this proof breakes if we conisder 2-disjoint sets. In this case one point of Ui1 can be on distance 1 to many components of Ui and the above inequality will not hold. But for 3-disjoint sets this works.



As for the proof of the lemma, I think, it should be rather standard.

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