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Algebra Difficulty 5.5 AIME, harder Prove it Belarus

Prove that there is no function f:RRf: \mathbb{R} \to \mathbb{R} such that f(f(x))=1xf(x)f(f(x)) = 1 - x f(x) for all real xx.
(I. Voronovich)

Solution

Suppose that there exists a function f:RRf: \mathbb{R} \to \mathbb{R} satisfying
f(f(x))=1xf(x)() f(f(x)) = 1 - x f(x) \quad (*)
for any xRx \in \mathbb{R}.

Set c=f(0)c = f(0). Using ()(*) for x=0x = 0, we obtain f(c)=f(f(0))=10f(0)=10c=1f(c) = f(f(0)) = 1 - 0 \cdot f(0) = 1 - 0 \cdot c = 1.
Since
1=f(c),(1) 1 = f(c), \quad (1)
we have f(1)=f(f(c))=1cf(c)=1c1=1cf(1) = f(f(c)) = 1 - c \cdot f(c) = 1 - c \cdot 1 = 1 - c. Similarly, since
1c=f(1),(2) 1 - c = f(1), \quad (2)
we have
f(1c)=f(f(1))=1f(1)=1(1c)=c.(3) f(1-c) = f(f(1)) = 1 - f(1) = 1 - (1-c) = c. \quad (3)
It follows that f(c)=f(f(1c))()=1(1c)f(1c)(3)=1(1c)c=c2c+1f(c) = f(f(1-c))_{(*)} = 1 - (1-c) \cdot f(1-c)_{(3)} = 1 - (1-c)c = c^2 - c + 1, i.e.
c2c+1=f(c).(4) c^2 - c + 1 = f(c). \quad (4)
From (1) and (4) it follows that c2c+1=1c^2 - c + 1 = 1, so either c=0c = 0 or c=1c = 1.
We show that both the equalities c=0c = 0 and c=1c = 1 give a contradiction. Indeed, if c=0c = 0, then from definition of cc it follows that 0=f(0)0 = f(0), but (1) gives 1=f(0)1 = f(0), a contradiction. If c=1c = 1, then (1) and (2) give 1=f(1)1 = f(1) and 0=f(1)0 = f(1) respectively, a contradiction. Therefore there is no function satisfying ()(*).

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