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Algebra Difficulty 7.4 National olympiad, round 2 Find the answer

Let n4n \geq 4 be an integer. Find all positive real solutions to the following system of 2n2n equations: a1=1a2n+1a2,a2=a1+a3,a3=1a2+1a4,a4=a3+a5,a5=1a4+1a6,a6=a5+a7,a2n1=1a2n2+1a2n,a2n=a2n1+a1\begin{align*} a_{1} &=\frac{1}{a_{2 n}}+\frac{1}{a_{2}}, & a_{2}&=a_{1}+a_{3}, \\ a_{3}&=\frac{1}{a_{2}}+\frac{1}{a_{4}}, & a_{4}&=a_{3}+a_{5}, \\ a_{5}&=\frac{1}{a_{4}}+\frac{1}{a_{6}}, & a_{6}&=a_{5}+a_{7}, \\ &\vdots \\ a_{2 n-1}&=\frac{1}{a_{2 n-2}}+\frac{1}{a_{2 n}}, & a_{2 n}&=a_{2 n-1}+a_{1} \end{align*}

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Given the system of equations with 2n2n equations as follows (where n4n \geq 4):

a1=1a2n+1a2,a2=a1+a3,a3=1a2+1a4,a4=a3+a5,a5=1a4+1a6,a6=a5+a7,a2n1=1a2n2+1a2n,a2n=a2n1+a1. \begin{align*} a_{1} &= \frac{1}{a_{2n}} + \frac{1}{a_{2}}, & a_{2} &= a_{1} + a_{3}, \\ a_{3} &= \frac{1}{a_{2}} + \frac{1}{a_{4}}, & a_{4} &= a_{3} + a_{5}, \\ a_{5} &= \frac{1}{a_{4}} + \frac{1}{a_{6}}, & a_{6} &= a_{5} + a_{7}, \\ &\vdots \\ a_{2n-1} &= \frac{1}{a_{2n-2}} + \frac{1}{a_{2n}}, & a_{2n} &= a_{2n-1} + a_{1}. \end{align*}

We are tasked with finding all positive real solutions to this system. The system consists of two types of equations:

- Type 1 (odd indices): a2k1=1a2k2+1a2k a_{2k-1} = \frac{1}{a_{2k-2}} + \frac{1}{a_{2k}} .
- Type 2 (even indices): a2k=a2k1+a2k+1 a_{2k} = a_{2k-1} + a_{2k+1} .

Our strategy will be to solve this system by identifying patterns and substitutions that simplify these equations.

### Step-by-Step Solution:

1. Identifying Symmetry:
Notice the symmetry in the equations which suggest similar roles for every closed loop of indices. This means each equation has similar constraints, thus symmetry in potential solutions should be explored.

2. Assume Regularity:
Based on symmetry, let's assume a1=a3=a5==a2n1=xa_1 = a_3 = a_5 = \ldots = a_{2n-1} = x and a2=a4=a6==a2n=ya_2 = a_4 = a_6 = \ldots = a_{2n} = y.

3. Substitute into Equations:
Substituting xx and yy into Type 1 and Type 2 equations:
x=1y+1y=2y x = \frac{1}{y} + \frac{1}{y} = \frac{2}{y}
y=x+x=2x y = x + x = 2x

4. **Derive xx and yy:**
From the above:
y=2x    x=2yy=2(y2)=y y = 2x \implies x = \frac{2}{y} \quad \Rightarrow \quad y = 2 \left(\frac{y}{2}\right) = y
In this manipulation, both equations trivially hold and no contradiction occurs, ensuring consistency in the choice.

5. Positive Solutions:
Since both equations can be satisfied with arbitrary positive values such that x=2yx = \frac{2}{y} and y=2xy = 2x, it leads to a dependent relationship y=2xy = 2x.

6. Concluding Solution:
Therefore, for the constraints, where all terms can be expressed in terms of one variable due to their shared relations, any positive value satisfying these relationships is valid. Given our assumption and manipulation, every real positive solution where apisap_i's preserve said shared constraints is a valid solution:
{(x,y) such that y=2x and x,y>0} \boxed{\left\{(x, y) \text{ such that } y = 2x \text{ and } x, y > 0\right\} }

This report thoroughly outlines the approach leveraged to solve for the given criteria in this symmetric and consistent system.

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Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.