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Algebra Difficulty 8.3 Shortlist Prove it Belarus

Given a function f(x)=x2+bx+cf(x) = x^2 + bx + c, where b,cRb, c \in \mathbb{R} and b0b \ge 0. Is it possible to represent the segment [0,1][0, 1] as the union ABA \cup B of two disjoint sets AA and BB such that f(A)=Bf(A) = B?
Recall that f(A)f(A) denotes the image of the set AA, that is, f(A)={f(a)aA}f(A) = \{f(a) \mid a \in A\}.

Solution

Answer: no. Suppose that the segment [0,1][0, 1] can be represented as the union ABA \cup B of two disjoint sets AA and BB such that f(A)=Bf(A) = B. Let us immediately note that the function ff is strictly increasing on the interval [0,1][0, 1] and is a bijection from [0,1][0, 1] to f([0,1])f([0, 1]).
Let us prove that ff has no fixed points on the interval [0,1][0, 1]. Indeed, let f(s)=sf(s) = s for some s[0,1]s \in [0, 1]. If sAs \in A, then f(s)=sBf(s) = s \notin B, and if sBs \in B, then there is no number from AA that is sent to ss, because ff is a bijection. Since both cases are impossible and [0,1]=AB[0, 1] = A \cup B, we arrive at a contradiction, that is, there cannot be fixed points. Due to the continuity of the function, this means that f(x)>xf(x) > x for all x[0,1]x \in [0, 1] or f(x)<xf(x) < x for all x[0,1]x \in [0, 1] depending on the sign of cc.
**Case c0c \ge 0.** We have x<f(x)x < f(x) for all x[0,1]x \in [0, 1]. Since f(0)=c>0f(0) = c > 0 and the function is increasing on the interval [0,1][0, 1], then f(x)cf(x) \ge c for any x[0,1]x \in [0, 1]. Therefore [0,c[A[0, c[ \subseteq A, whence [c,f(c)][B[c, f(c)][ \subseteq B, and c<f(c)c < f(c). Let us now consider the interval [f(c),f(f(c))[[f(c), f(f(c))[. Since all points of this interval are images of points from BB and not from AA (after all, ff is a bijection), the intersection of [f(c),f(f(c))[[f(c), f(f(c))[ with BB is empty. We know that f(c)1f(c) \le 1 and f(c)<f(f(c))f(c) < f(f(c)). Note that 1[f(c),f(f(c))[1 \notin [f(c), f(f(c))[, because otherwise we would get that 1B1 \notin B, whence 1A1 \in A and 1<f(1)B1 < f(1) \in B, which is impossible. Consequently, [f(c),f(f(c))[[f(c), f(f(c))[ A\subseteq A and, continuing the reasoning in this way, we obtain that the sequence
0,f(0),f2(0),f3(0),0, f(0), f^2(0), f^3(0), \dots
increases and lies entirely in [0,1][0, 1], that is, it is bounded above. This means that it has a limit [0,1]\ell \in [0, 1]. From the continuity of ff we obtain that
=limnfn(0)=f(limnfn1(0))=f(), \ell = \lim_{n \to \infty} f^n(0) = f\left(\lim_{n \to \infty} f^{n-1}(0)\right) = f(\ell),
that is, \ell is a fixed point. Contradiction.
**Case c<0c < 0.** We have f(x)<xf(x) < x for all x[0,1]x \in [0, 1]. In particular, f(1)<1f(1) < 1. The function is increasing on the interval [0,1][0, 1], and so f(x)f(1)f(x) \le f(1) for any x[0,1]x \in [0, 1]. Therefore ]f(1),1]A]f(1), 1] \subseteq A, whence ]f(f(1)),f(1)]B]f(f(1)), f(1)] \subseteq B, and f(f(1))<f(1)f(f(1)) < f(1). Similar to the previous case, we show that ]f3(1),f2(1))[]f^3(1), f^2(1))[ A\subseteq A and that the sequence
1,f(1),f2(1),f3(1), 1, f(1), f^2(1), f^3(1), \dots
converges to some number [0,1]\ell \in [0, 1], which is a fixed point for ff. Contradiction.

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