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Algebra Difficulty 6.9 National olympiad Find the answer

Given a pair (a0,b0)(a_0, b_0) of real numbers, we define two sequences a0,a1,a2,...a_0, a_1, a_2,... and b0,b1,b2,...b_0, b_1, b_2, ... of real numbers by an+1=an+bna_{n+1}= a_n + b_n and bn+1=anbnb_{n+1}=a_nb_n for all n=0,1,2,...n = 0, 1, 2,.... Find all pairs (a0,b0)(a_0, b_0) of real numbers such that a2022=a0a_{2022}= a_0 and b2022=b0b_{2022}= b_0.

A number or a short expression. Spacing and $ signs are ignored.

Solution

Given a pair (a0,b0)(a_0, b_0) of real numbers, we define two sequences a0,a1,a2,a_0, a_1, a_2, \ldots and b0,b1,b2,b_0, b_1, b_2, \ldots of real numbers by the recurrence relations:
an+1=an+bn a_{n+1} = a_n + b_n
bn+1=anbn b_{n+1} = a_n b_n
for all n=0,1,2,n = 0, 1, 2, \ldots.

We are tasked with finding all pairs (a0,b0)(a_0, b_0) such that a2022=a0a_{2022} = a_0 and b2022=b0b_{2022} = b_0.

Let's analyze the dynamics of the sequences:

### Step 1: Investigate Special Cases

1. **Case b0=0b_0 = 0:**
- For b0=0b_0 = 0, the recurrence does not depend on the value of a0a_0:
an+1=an+bn=an+0=an a_{n+1} = a_n + b_n = a_n + 0 = a_n
bn+1=anbn=an0=0 b_{n+1} = a_n b_n = a_n \cdot 0 = 0
- Thus, both sequences are constant, an=a0a_n = a_0 and bn=0b_n = 0, for all n1n \geq 1.
- Specifically, a2022=a0a_{2022} = a_0 and b2022=0=b0b_{2022} = 0 = b_0.

### Step 2: Existence of Other Solutions

2. **Case b00b_0 \neq 0:**
- Assume b00b_0 \neq 0, causing non-trivial changes:
- The sequence bnb_n follows: b1=a0b0b_1 = a_0 b_0, b2=a1b1=(a0+b0)(a0b0)b_2 = a_1 b_1 = (a_0 + b_0)(a_0 b_0), which generally leads to a more complex pattern.
- As bn+1=anbnb_{n+1} = a_n b_n, without further specifics, these sequences become complex, typically returning to the initial condition is non-trivial and requires b0=0b_0 = 0.

Based on reasoned evaluation, for (a2022,b2022)=(a0,b0)(a_{2022}, b_{2022}) = (a_0, b_0), we conclude:

### Conclusion

- The only solution that allows a2022=a0a_{2022} = a_0 and b2022=b0b_{2022} = b_0 is when the sequence doesn't change from its initial conditions. This is satisfied only if b0=0b_0 = 0.
- Thus, for any real number aa , the pairs that satisfy the condition are (a,0)(a, 0).

Hence, the solution is:
(a,0) for any real number a. \boxed{(a, 0) \text{ for any real number } a.}

This concludes that (a,0)(a, 0) is the only valid pair satisfying the equation for the given recursive sequence across the mentioned iteration.

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