Math Is Fun Forum

luca-deltodesco

Member

Registered: 2006-05-05

Posts: 1,470

False proofs

As a side-thread to the interesting proofs one, this is for interesting proofs, that are actually flawed.

3 = 1

flaw

Natural logarithm is only one-one over the reals, over the complex numbers, it is a multi-valued function, so the step is not valid.

Last edited by luca-deltodesco (2007-02-04 05:39:42)

---

The Beginning Of All Things To End.
The End Of All Things To Come.

Dross

Member

Registered: 2006-08-24

Posts: 325

Re: False proofs

Conjecture:

Everybody in the UK has ginger hair.

Proof:

This will be a proof by induction. Let P(n) be the statement: "n people in the world have ginger hair".

Now we show P(n). Clearly we can find one person in the world who is ginger.

Now we assume that if P(n) is true, P(n+1) is true. Get any n+1 people into the UK, get everybody else out. Now, remove one of the people. Now, since P(n) is true (by our assumption), we know that all the n people in the UK are ginger. Now bring the excluded person back in and send a different person out. Again, P(n) is true so all these people are ginger-haired. Bring back the excluded person and we have n+1 people in the UK, all of whom must be ginger.

Therefore, P(n) is true for n arbitrarily large. Specifically, we can make it as large as the number of people in the UK.

So, by induction, all people in the UK are ginger.

---

Bad speling makes me [sic]

Sekky

Member

Registered: 2007-01-12

Posts: 181

Re: False proofs

The flaw is you assume P(n) -> P(n+1), which is the proof requisite. It's circular