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Algebra Difficulty 8.5 Shortlist Prove it Vietnam

Given three functions
P(x)=(x2−1)2023,Q(x)=(2x+1)14,R(x)=(2x+1+2x)34. P(x) = (x^2 - 1)^{2023}, Q(x) = (2x + 1)^{14}, R(x) = \left(2x + 1 + \frac{2}{x}\right)^{34}.
Initially, we pick a set SS containing two of these functions, and we perform some operations on it. Allowed operations include:
* Take two functions p,q∈Sp, q \in S and add one of p+q,p−qp+q, p-q, or pqpq to SS.
* Take a function p∈Sp \in S and add pkp^k to SS for kk is an arbitrary positive integer.
* Take a function p∈Sp \in S and choose a real number tt, and add to SS one of the function p+t,p−t,ptp+t, p-t, pt.
Show that no matter how we pick SS in the beginning, there is no way we can perform finitely many operations on SS that would eventually yield the third function not in SS.

Solution

First of all, from P(x),Q(x)P(x), Q(x), after all of the operations, we get a polynomial variable xx, in which R(x)R(x) is not a polynomial, so it is not possible to have
P,Q→R. P, Q \to R.
Next, we will show that R,P→QR, P \to Q cannot be obtained. Indeed,
P′(x)=4046x(x2−1)2022,R′(x)=34(2−2x2)(2x+1+2x)33 P'(x) = 4046x(x^2-1)^{2022}, R'(x) = 34\left(2-\frac{2}{x^2}\right)\left(2x+1+\frac{2}{x}\right)^{33}
so obviously P′(±1)=R′(±1)=0P'(\pm 1) = R'(\pm 1) = 0. The given operations will generate functions that are composite functions of P(x),R(x)P(x), R(x) so for each of those functions, if taking the derivative, they still have a solution of ±1\pm 1. However, Q(x)Q(x) does not satisfy this because
Q′(x)=14⋅2⋅(2x+1)13. Q'(x) = 14 \cdot 2 \cdot (2x+1)^{13}.
Finally, we will show that R,Q→PR, Q \to P cannot be obtained using polynomial congruence, namely by considering modulo f(x)=x2+x+1f(x) = x^2+x+1. Next, if P(x)−Q(x)P(x) - Q(x) is divisible by f(x)f(x), we denote
P(x)≡Q(x)mod  f(x). P(x) \equiv Q(x) \mod f(x).
It is easy to check that the properties of the integer congruence are still true here. From this, we can also define congruence for fractions. Back to the problem, we have
Q(x)=(2x+1)14=(4x2+4x+1)7≡(−3)7(modf(x)), Q(x) = (2x+1)^{14} = (4x^2+4x+1)^7 \equiv (-3)^7 \pmod{f(x)},
R(x)=(2(x2+1)x+1)34≡(−2+1)34≡1(modf(x)). R(x) = \left(\frac{2(x^2+1)}{x} + 1\right)^{34} \equiv (-2+1)^{34} \equiv 1 \pmod{f(x)}.
Therefore, all functions generated by Q,RQ, R, when taking congruent modulo f(x)f(x) are constant. We show that P(x)P(x) does not have that property. Indeed, notice that x6≡1(modf(x))x^6 \equiv 1 \pmod{f(x)} so
(x2−1)2≡(−x−2)2≡x2+4x+4≡x2−4x2≡−3x2, (x^2 - 1)^2 \equiv (-x - 2)^2 \equiv x^2 + 4x + 4 \equiv x^2 - 4x^2 \equiv -3x^2,
(x2−1)6≡(−3x2)3=−27x6≡−27(modf(x)). (x^2 - 1)^6 \equiv (-3x^2)^3 = -27x^6 \equiv -27 \pmod{f(x)}.
So that
P(x)≡(x2−1)2022⋅(x2−1)≡(−27)337⋅(−x−2)=27337(x+2)≠const mod f(x). P(x) \equiv (x^2 - 1)^{2022} \cdot (x^2 - 1) \equiv (-27)^{337} \cdot (-x - 2) \\ = 27^{337}(x + 2) \neq \text{const mod } f(x).
The problem is completely solved. □

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