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Why the Golden Ratio and not -1/the Golden Ratio?

Take any real number and call it x. Then plug it into the equation f(x) = 1 + 1/x and keep doing it many times in a row, plugging the result back into the equation.

At some point you will see that you arrive at a value that will become stable and not change anymore. And that value will be… φ, the golden ratio!

But this equation also has another answer, -1/φ. If you plug that value into the equation, it will be the same, too. The real magic happens once you have rounded the -1/φ down (or up), i.e. once what you plug into the equation is no longer exactly -1/φ. What happens is that, if you keep going, you will eventually reach… φ as your answer!

Simon saw this interesting fact in a video by 3Blue1Brown and then came up with a proof as to why it happens.

Simon also sketched his proof in GeoGebra: https://www.geogebra.org/classic/zxmqdspb

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Simon’s proof that Phi is not transcendental

Simon has come up with a proof that Phi (the Golden Ratio) is an algebraic number (not transcendental). He proves it by showing that Phi can be the solution to a polynomial equation (which would be impossible if it was a transcendental number). Indeed, if you simplify Simon’s polynomial further, you can get x squared minus x equals one, which describes one of Phi’s remarkable qualities: the square of Phi (an infinite irrational fraction) equals exactly Phi plus 1. In fact, Simon has talked about this in his previous video (expressing Fibonacci sequence using Lucas Numbers):

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Simon’s Fibonacci function and Fibonacci counter in p5.js

Simon came up with this Fibonacci function while taking a walk downtown:

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f(0) = 0

f(1) = 1

f(n) = f(n-1)+f(n-2)

When we got home, he used the function to build a Fibonacci counter in p5.js:

You can play with Simon’s Fibonacci counter online at: https://alpha.editor.p5js.org/simontiger/sketches/Skhr3o8Gf

The idea about the Fibonacci function struck Simon when he was looking down at the cobbles under his feet. “Look, Mom! It’s a golden rectangle!”, he shouted:

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He had read that golden ratio has a direct connection to the Fibonacci sequence. The same evening, he took out his compasses to draw a golden rectangle (this time not his own invention, but following the steps from his Murderous Math book):

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If you turn the page, the smaller rectangle is a golden rectangle as well, and if you slice a square off of it, the remaining rectangle will also have the golden proportions. You can continue doing this infinitely. The sizes of the rectangles will exactly correspond to the numbers in the Fibonacci sequence, which makes these drawings an illustration to the sequence.

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The next day, Simon showed his function to his math teacher. Below are the Fibonacci sequence numbers he got through his selfmade JavaScript program. After a certain number, the computer started taking too long to compute the following number in the sequence (several seconds per number), but didn’t crash.

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