Maps with Bounded Stretch

Problem

Let A={0,1,2,3,4,5,6}A = \{0, 1, 2, 3, 4, 5, 6\}, and let ff be a function from AA to the integers such that f(0)=0f(0) = 0, f(6)=12f(6) = 12, and

xyf(x)f(y)3xyfor all x,yA.|x - y| \leqslant |f(x) - f(y)| \leqslant 3|x - y| \quad \text{for all } x, y \in A.

Find the number of such functions ff.

Answer

Solution

Difficulty6/10
Topicscombinatorics, functions, Counting, Casework

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