MHB Solve Odd Math Puzzle: a^2+b^2=k(ab+1)

  • Thread starter Thread starter mathpleb
  • Start date Start date
AI Thread Summary
The discussion revolves around the math puzzle a^2 + b^2 = k(ab + 1), where a and b are positive integers. The main question is whether k can only be a fraction or a perfect square, with the initial assertion being that k is always a fraction. It is clarified that the equation should be expressed as k = (a^2 + b^2) / (1 + ab) and the task is to show that if k is an integer, it must be a perfect square. The proof and its historical context are referenced in a Wikipedia article on Vieta jumping. The thread emphasizes the mathematical implications of the equation and its connection to a specific problem from the 1988 International Mathematical Olympiad.
mathpleb
Messages
1
Reaction score
0
This isn't for me, it's for a friend. I'm still teaching myself stuff before Trigonometry.

Anyways, he has a puzzle. a^2+b^2=k(ab+1).

A and B are given as positive integers.

Q: "Prove that K can only take on the value of fractions or squares."
 
Mathematics news on Phys.org
Isn't this clear, since $k = \frac{a^2 + b^2}{ab + 1}$ is always a fraction when $a$ and $b$ are positive integers? Or perhaps I am overlooking something?
Also, could it be you posted this accidentally in the "differential equations" section?

EDIT: I see it was already moved to the right section, thank you (Smile)
 
I suspect this is supposed to be problem #6 at IMO 1988, in which case it should read:

Let $\displaystyle k={{a^2+b^2}\over{1+ab}}.
$ Show that if $k$ is an integer then $k$ is a perfect square.

The proof (along with its history) is given in the Wikipedia article on Vieta jumping.
 
Suppose ,instead of the usual x,y coordinate system with an I basis vector along the x -axis and a corresponding j basis vector along the y-axis we instead have a different pair of basis vectors ,call them e and f along their respective axes. I have seen that this is an important subject in maths My question is what physical applications does such a model apply to? I am asking here because I have devoted quite a lot of time in the past to understanding convectors and the dual...
Insights auto threads is broken atm, so I'm manually creating these for new Insight articles. In Dirac’s Principles of Quantum Mechanics published in 1930 he introduced a “convenient notation” he referred to as a “delta function” which he treated as a continuum analog to the discrete Kronecker delta. The Kronecker delta is simply the indexed components of the identity operator in matrix algebra Source: https://www.physicsforums.com/insights/what-exactly-is-diracs-delta-function/ by...
Back
Top