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It's one of my real analysis professor's favourite sayings that "being obvious does not imply that it's true".

Now, I know a fair few examples of things that are obviously true and that can be proved to be true (like the Jordan curve theorem).

But what are some theorems (preferably short ones) which, when put into layman's terms, the average person would claim to be true, but, which, actually, are false (i.e. counter-intuitively-false theorems)?

The only ones that spring to my mind are the Monty Hall problem and the divergence of $\sum\limits_{n=1}^{\infty}\frac{1}{n}$ (counter-intuitive for me, at least, since $\frac{1}{n} \to 0$ ).

I suppose, also, that $$\lim\limits_{n \to \infty}\left(1+\frac{1}{n}\right)^n = e=\sum\limits_{n=0}^{\infty}\frac{1}{n!}$$ is not obvious, since one 'expects' that $\left(1+\frac{1}{n}\right)^n \to (1+0)^n=1$.

I'm looking just for theorems and not their (dis)proof -- I'm happy to research that myself.


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The Banach-Tarski paradox is a good one. –  mjqxxxx Jun 4 '14 at 18:08
related: mathoverflow.net/questions/23478/… –  Will Jagy Jun 4 '14 at 18:29
This question is hard to answer, because an essential skill for a mathematician is being able to rapidly retune your intuition to match the truth. So once you know that a fact is false, very soon it no longer seems obvious. –  Nate Eldredge Jun 4 '14 at 19:53
I strongly disagree that the Jordan Curve theorem is obviously true! I would agree that the Jordan Curve theorem for piecewise smooth curves is obviously true; but it's also pretty easy to prove, at least compared to the topological version. (Is it really obvious that the Koch snowflake doesn't have some pathological path from the inside to the outside?) –  Mike Miller Jun 6 '14 at 7:04
"Every true statement can be proved" –  sinelaw Jun 9 '14 at 17:27

64 Answers 64

If a propositional calculus A contains all theorems of propositional calculus B under detachment and uniform substitution for propositional variables, but B does not contain all of the theorems of A, then one of the shortest single axioms of A is longer than any of the shortest single axioms of B. Or one might more haphazardly say "if propositional calculus A is bigger than propositional calculus B, then one of the shortest single axioms of A is longer than any of the shortest single axioms of B."

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Good luck finding a layman whose eyes don't glaze at the term "propositional calculus", let alone one who finds something here obvious. –  rumtscho Jun 13 '14 at 17:46

An analytic function with compact support vanishes identically.

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That is actually true, as long as we stay in $\mathbb{C}^n$ resp. $\mathbb{R}^n$. It becomes false when one takes a compact analytic manifold, but then it's obvious that it's false. –  Daniel Fischer Jun 4 '14 at 19:45

For me a nice example of all of "evidence" suggesting it was true is

$$\pi(x) < \operatorname{li}(x)$$

until Skewes showed that $\pi(x) - \operatorname{li}(x)$ changes sign infinitely often

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The question asks for obvious but false statements. Are you seriously claiming that $\pi(x) < \operatorname{li}(x)$ is obvious? –  MJD Jun 5 '14 at 21:32
hard to tell whether that is supposed to be obvious... What are $\pi(x)$ and $\operatorname{li}(x)$? –  example Jun 5 '14 at 22:03

I don't know how layman the laymen you are talking about are, but if you are trying to amaze someone with elementary knowlegde of maths, just try some simple things like:

  1. The $1=2$ proof
  2. A set can be open and closed at the same time
  3. Every section of the number line (interval) has the same number of points.
  4. There are just as many even numbers as natural numbers. (Three and four can even be done with twelve year olds if you make a good drawing.)
  5. The fact that a heavier object doesn't fall faster than a light one in a free fall
  6. The number of ways to pick $9$ nine persons out of $10$ is the same as picking $1$ out of $10$

Some of these seem really basic, but you asked about things that can be put in laymen's terms and I did not see a lot of answers that didn't at least required a (almoast) firs year undergraduate knowledge of maths.

Anyway, not sure if this is exactly what you are looking for, but I thought I'd post it anyway. Just trying to help :)

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Most of these are not answers to the question, which is for claims that appear obviously true, but which are false. For example, the claim that $1=2$ is not obviously true; it appears false, and it is false. The claim that a set can be open and closed at the same time hinges on the very specific technical definitions of "open" and "closed", which is generally agreed to be a poor choice of terminology. I think most people would agree that #6 was obviously true, but it is true. To make #5 true you have to consider free fall in a vacuum, at which point it is no longer obvious. –  MJD Jun 6 '14 at 14:56

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