Maths Olympiad Prep

Library / /7 of 144

Algebra Difficulty 7.5 National olympiad, round 2 Find the answer

Let a0a_0, a1a_1, a2a_2, ... be an infinite sequence of real numbers satisfying the equation an=an+1an+2a_n=\left|a_{n+1}-a_{n+2}\right| for all n0n\geq 0, where a0a_0 and a1a_1 are two different positive reals.

Can this sequence a0a_0, a1a_1, a2a_2, ... be bounded?

*

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

Solution

We have an infinite sequence of real numbers (an)n0(a_n)_{n \geq 0} satisfying the recursive relation:

an=an+1an+2 a_n = \left|a_{n+1} - a_{n+2}\right|

for all n0n \geq 0, where a0a_0 and a1a_1 are two different positive real numbers. Our goal is to determine whether this sequence can be bounded.

### Step 1: Analyze the recursive relation
The given relation

an=an+1an+2 a_n = \left|a_{n+1} - a_{n+2}\right|

implies a functional dependency between three consecutive terms of the sequence. Importantly, since ana_n is defined as the absolute difference between an+1a_{n+1} and an+2a_{n+2}, the sequence heavily depends on the initial values a0a_0 and a1a_1.

### Step 2: Investigate the effect of positive differences
Since a0a_0 and a1a_1 are given as different positive real numbers, we can start by observing:

- If a0=a1a2a_0 = \left|a_1 - a_2\right| and a0>0a_0 > 0, then either a1>a2a_1 > a_2 or a2>a1a_2 > a_1.
- Similarly, a1=a2a3a_1 = \left|a_2 - a_3\right| and a1>0a_1 > 0 implies that a2a3a_2 \neq a_3.

### Step 3: Recursive impact of differing terms
Because a0a_0 and a1a_1 are distinct, as the sequence progresses:

- If an+1=an+2a_{n+1} = a_{n+2}, then an=0a_n = 0, which cannot sustain for all nn since the sequence starts with non-zero values.
- Given the recursive nature, each ana_n serves as a source for subsequent terms, making it improbable for all terms to converge to a single bound due to iterative differences.

### Step 4: Sequence dynamics based on initial conditions
Given that a0a_0 and a1a_1 are positive and different, the sequence infinitely alternates in such a manner that it is impossible for it to stabilize or converge to a bound. The discrepancy between consecutive elements will propagate due to the nature of the absolute difference, thereby causing an unbounded progression along the sequence.

#### Conclusion
Based on the conditions and recursive relation, the sequence (an)(a_n) cannot be bounded, implying that it will diverge or fluctuate indefinitely without settling within any finite bounds.

Therefore, the sequence is not bounded:

No \boxed{\text{No}}

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

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