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I am not sure how appropriate this question is for MO. If it is not, I apologize in advance but I could not resist asking it and if by any chance I get some interesting answers, it will for sure be very useful to keep my students excited about mathematics and physics as September arrives.

We all know very well that $\pi$ (the ratio of the circumference of a circle to its diameter in Euclidean space) is irrational and even transcendental. These are some of the famous results in all mathematics.

So I was wondering what will go wrong if $\pi$ was just an integer number?

Are there important theorems that are based on the fact that it is actually irrational and/or transcendental?

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closed as not a real question by Pete L. Clark, Andrea Ferretti, Felipe Voloch, Loop Space, Robin Chapman Aug 12 '10 at 16:42

It's difficult to tell what is being asked here. This question is ambiguous, vague, incomplete, overly broad, or rhetorical and cannot be reasonably answered in its current form. For help clarifying this question so that it can be reopened, visit the help center.If this question can be reworded to fit the rules in the help center, please edit the question.

Is it the presence of the word "universe" in the title of the question which justifies the mathematical physics tag?! – José Figueroa-O'Farrill Aug 12 '10 at 12:01
Sorry JME, I don't think this question is suitable for MathOverflow. Please, have a look at the FAQ. Maybe you will find there somewhere else where to post this (although I would not know what to suggest). – Andrea Ferretti Aug 12 '10 at 12:03
Somewhat related: – José Figueroa-O'Farrill Aug 12 '10 at 12:04
I've started a meta thread to discuss closing (or not) this question:… – José Figueroa-O'Farrill Aug 12 '10 at 12:13
I have a funny feeling about this question, that there's a fantastic answer lurking out there somewhere. – Rob Grey Aug 12 '10 at 13:54

Since Euler showed that $$\frac{\pi^2}{6}=\prod_{p} \Big(1-\frac{1}{p^2}\Big)^{-1},$$ the fact that $\pi^2$ is irrational implies that there are infinitely many primes.

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Gregory's series's_series for $\pi/4$ also has an Euler product, so the irrationality of $\pi$ itself shows the infinitude of primes. – Stopple Aug 12 '10 at 20:07

The fact that π is irrational has few direct applications. However the techniques used to prove this, or rather used to prove the stronger statement that it is transcendental, have many applications. For example, Baker proved that 1 and the logs of algebraic numbers are linearly independent over algebraic numbers except in trivial cases. (This includes the fact that π is irrational as a special case because π = log(-1)/i.) Baker used his theorem to give effective bounds on the solutions of Diophantine equations and to solve Gauss's class number problem for imaginary quadratic fields, among other things. See Baker's book on transcendental number theory for more details.

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The fact that one cannot square the circle was proven as a corollary of the fact that pi is transcendental.

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thanks, but I was expecting less obvious examples – JME Aug 12 '10 at 14:05
I was about to write the same answer. @JME: Next time, you will be probably better off if you include the facts/examples which you are aware of in your question. It is not very nice to reply “that’s obvious” to a person who answered your question. – Tsuyoshi Ito Aug 12 '10 at 15:21
@Tsuyoshi yes, you are absolutely right. Johan, I am deeply sorry for my heartless comment, it was not in bad spirit. – JME Aug 12 '10 at 16:46
No offense taken. – Johan Aug 18 '10 at 20:13

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