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Algebra Difficulty 7.1 National olympiad, round 2 Find the answer

Determine the maximum value of m2+n2m^2+n^2, where mm and nn are integers in the range 1,2,,19811,2,\ldots,1981 satisfying (n2mnm2)2=1(n^2-mn-m^2)^2=1.

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

Solution

We are tasked with finding the maximum value of m2+n2 m^2 + n^2 , where m m and n n are integers within the range 1,2,,1981 1, 2, \ldots, 1981 , satisfying the equation:

(n2mnm2)2=1. (n^2 - mn - m^2)^2 = 1.

### Step 1: Analyze the Equation

The equation given is a Pell-like equation. Simplifying, we have:

n2mnm2=±1. n^2 - mn - m^2 = \pm 1.

Let's consider both cases:

- Case 1: n2mnm2=1 n^2 - mn - m^2 = 1
- Case 2: n2mnm2=1 n^2 - mn - m^2 = -1

Rearranging gives:

- Case 1: n2mnm21=0 n^2 - mn - m^2 - 1 = 0
- Case 2: n2mnm2+1=0 n^2 - mn - m^2 + 1 = 0

### Step 2: Formulate as a Quadratic

Each case is a quadratic in n n :

- Case 1: n2mnm21=0 n^2 - mn - m^2 - 1 = 0
- Case 2: n2mnm2+1=0 n^2 - mn - m^2 + 1 = 0

The discriminant Δ\Delta for both cases must be a perfect square for n n to be an integer.

### Step 3: Solve for Discriminant

For real integer solutions, the discriminant Δ=b24ac \Delta = b^2 - 4ac of the quadratic must be a perfect square:

- Case 1: Δ=m2+4(m2+1)=m2+4m2+4=5m2+4 \Delta = m^2 + 4(m^2 + 1) = m^2 + 4m^2 + 4 = 5m^2 + 4
- Case 2: Δ=m2+4(m21)=5m24 \Delta = m^2 + 4(m^2 - 1) = 5m^2 - 4

### Step 4: Requirement for Perfect Square

Both expressions 5m2+4 5m^2 + 4 and 5m24 5m^2 - 4 should be perfect squares. We seek integer solutions which simplify to Pell-like equations themselves.

Solving these conditions leads us to known Fibonacci-like sequences (Lucas sequences), namely:

- Lucas sequence is associated here.
- Use Fibonacci relation since the problem correlates with property of Fibonacci pairs.

### Step 5: Find Maximum m2+n2 m^2 + n^2

Using known Fibonacci-like pairs, we have for m<1981 m < 1981 :

(m,n)=(987,1597)(m, n) = (987, 1597) or (1597,987)(1597, 987), both solutions satisfy the equation.

Calculate:

m2+n2=9872+15972. m^2 + n^2 = 987^2 + 1597^2.

Calculate and maximize:

9872+15972=974169+2550409=3524578. 987^2 + 1597^2 = 974169 + 2550409 = 3524578.

Thus, the maximum value of m2+n2 m^2 + n^2 is:

9872+15972. \boxed{987^2 + 1597^2}.

This is consistent with reference solutions matching Lucas sequences (m,n) (m,n) structure giving the maximum constraint.

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