3 squares 4 squares

Algebra Level 5

Let $a_1,a_2,...,a_n$ be different positive integers and let $1 \le m \le n$ be a positive integer.

Find the sum of all $n$ so that for all $1 \le m \le n$ , there does not exist $a_1,...,a_n$ satisfying $a_1^2+a_2^2+...+a_m^2=a_{m+1}^2+...+a_n^2$ .

Note: This idea is attributed to Mohammed Imran .

1 solution

Steven Jim
Mar 26, 2020

Since I'm too lazy to type down my solution I will give the main idea.

Step 1: Prove that there are infinitely many Pythagorean triples (effectively proving $n=3$ is viable).

Step 2: Prove that there are infinitely many Pythagorean quadruples (effectively proving $n=4$ is viable).

Step 3: Show that for $n<3$ there exists no solutions.

Step 4: Show that for $n=5$ there exists a solution. (It's better to prove that there exists infinitely many solutions, but it's hard.)

Step 5: Show that for $n \ge 6$ you can always create an answer, using Steps 1 and 2.

That's all.