Contractions and expansion
Abstract
Let A be a finite set of reals and let K >= 1 be a real number. Suppose that for each a in A we are given an injective map fa : A -> R which fixes a and contracts other points towards it in the sense that |a - fa(x)| <= |a - x|/K for all x in A, and such that fa(x) always lies between a and x. Then the union of the fa(A) has cardinality >= K|A|/10 - OK(1). An immediate consequence of this is the estimate |A + K.A| >= K|A|/10 - OK(1), which is a slightly weakened version of a result of Bukh.
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