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Combinatorics Difficulty 7.0 National olympiad Prove it Mongolia

nn is a positive integer and relatively prime with 66. a1,,an,b1,,bna_1, \dots, a_n, b_1, \dots, b_n are positive integers such that a1<a2<<ana_1 < a_2 < \dots < a_n and b1<b2<<bnb_1 < b_2 < \dots < b_n. If for arbitrary natural number tt such that ai+aj+ak=ta_i + a_j + a_k = t (i<j<ki < j < k) triple's number is equal to bi+bj+bk=tb_i + b_j + b_k = t (i<j<ki < j < k) then prove that a1=b1,a2=b2,,an=bna_1 = b_1, a_2 = b_2, \dots, a_n = b_n.
(proposed by G. Batzaya)

Solution

Consider the following polynomials:
a(x)=xa1+xa2++xan a(x) = x^{a_1} + x^{a_2} + \dots + x^{a_n}
b(x)=xb1+xb2++xbn b(x) = x^{b_1} + x^{b_2} + \dots + x^{b_n}
Now assume the contrary, in other words a(x)b(x)a(x) \neq b(x).
[a(x)]3=(i=1nxai)(ijxai+aj+i=1nx2ai)==(i=1nxai)(ijxai+aj)+(ijxai)(i=1nx2ai)==(ijkxai+aj+ak+2ijxai+2aj)+(ijxai+2aj+i=1nx3ai)(ijkxai+aj+ak)=[a(x)]33a(x)a(x2)+2a(x3) \begin{aligned} [a(x)]^3 &= \left(\sum_{i=1}^{n} x^{a_i}\right) \cdot \left(\sum_{i \neq j} x^{a_i+a_j} + \sum_{i=1}^{n} x^{2a_i}\right) = \\ &= \left(\sum_{i=1}^{n} x^{a_i}\right) \left(\sum_{i \neq j} x^{a_i+a_j}\right) + \left(\sum_{i \neq j} x^{a_i}\right) \left(\sum_{i=1}^{n} x^{2a_i}\right) = \\ &= \left(\sum_{i \neq j \neq k} x^{a_i+a_j+a_k} + 2 \sum_{i \neq j} x^{a_i+2a_j}\right) + \left(\sum_{i \neq j} x^{a_i+2a_j} + \sum_{i=1}^{n} x^{3a_i}\right) \\ & \qquad \left(\sum_{i \neq j \neq k} x^{a_i+a_j+a_k}\right) = [a(x)]^3 - 3a(x) \cdot a(x^2) + 2a(x^3) \end{aligned}

Analogously, we have [b(x)]3=[b(x)]33b(x)b(x2)+2b(x3)[b(x)]^3 = [b(x)]^3 - 3b(x) \cdot b(x^2) + 2b(x^3). From the given condition, we get the following equality:
[a(x)]33a(x)a(x2)+2a(x3)=[b(x)]33b(x)b(x2)+2b(x3)() [a(x)]^3 - 3a(x) \cdot a(x^2) + 2a(x^3) = [b(x)]^3 - 3b(x) \cdot b(x^2) + 2b(x^3) \quad (*)

Now consider f(x)=a(x)b(x)f(x) = a(x) - b(x) polynomials. Therefore f(1)=a(1)b(1)=nn=0f(1) = a(1) - b(1) = n - n = 0. Thus we can write ff the following way:
f(x)=(x1)kh(x),h(1)0() f(x) = (x-1)^k \cdot h(x), \quad h(1) \neq 0 \qquad (**)
Also, by (*) we have
(x1)kh(x)([a(x)]2+a(x)b(x)+[b(x)]2)3(a(x)(x21)kh(x2)+b(x2)(x1)kh(x))++2(x31)kh(x3)=0. (x-1)^k \cdot h(x) ([a(x)]^2 + a(x) \cdot b(x) + [b(x)]^2) - \\ -3 \left( a(x)(x^2 - 1)^k \cdot h(x^2) + b(x^2)(x-1)^k \cdot h(x) \right) + \\ +2 \cdot (x^3 - 1)^k \cdot h(x^3) = 0.
The last equation is the same as the following:
h(x)([a(x)]2+a(x)b(x)+[b(x)]2)3(a(x)(x+1)kh(x2)+b(x2)h(x))++2(x2+x+1)kh(x3)=0.() h(x) ([a(x)]^2 + a(x) \cdot b(x) + [b(x)]^2) - \\ -3 \left( a(x) \cdot (x+1)^k \cdot h(x^2) + b(x^2) \cdot h(x) \right) + \\ +2 \cdot (x^2 + x + 1)^k \cdot h(x^3) = 0. \qquad (***)
In the equation (***) substituting x=1x = 1, we get 3n23(n2k+n)+23k=03n^2 - 3(n \cdot 2^k + n) + 2 \cdot 3^k = 0 which is the same as n2n(2k+1)+23k1=0n^2 - n \cdot (2^k + 1) + 2 \cdot 3^{k-1} = 0. Therefore, this implies that n23k1n|2 \cdot 3^{k-1}. This is a contradiction with (n,6)=1(n, 6) = 1.

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