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Affine and convex functions

Source: Simon Marais Mathematics Competition 2023 Paper A Problem 2

October 14, 2023
functionalgebra

Problem Statement

Let nn be a positive integer and let f1(x),f2(x)fn(x)f_1(x), f_2(x) \dots f_n(x) be affine functions from R\mathbb{R} to R\mathbb{R} such that, amongst the graph of these functions, no two are parallel and no three are concurrent. Let SS be the set of all convex functions g(x)g(x) from R\mathbb{R} to R\mathbb{R} such that for each xRx \in \mathbb{R}, there exists ii such that g(x)=fi(x)g(x) = f_i(x).
Determine the largest and smallest possible values of S|S| in terms of nn.
(A function f(x)f(x) is affine if it is of form f(x)=ax+bf(x) = ax + b for some a,bRa, b \in \mathbb{R}. A function g(x)g(x) is convex if g(λx+(1λ)y)λg(x)+(1λ)g(y)g(\lambda x + (1 - \lambda) y) \leq \lambda g(x) + (1-\lambda)g(y) for all x,yRx, y \in \mathbb{R} and 0λ10 \leq \lambda \leq 1)