[FOM] Historical queries

joeshipman@aol.com joeshipman at aol.com
Wed Nov 14 10:26:33 EST 2007

I wrote:

>-----Original Message-----
>From: joeshipman at aol.com
>In a forthcoming paper cowritten with John H. Conway, I show that, for
>any non-square integer N, there is a very simple geometric proof that
>the square root of N is irrational that could have been demonstrated 
>Theodorus; however, to find the proof requires finding a number of the
>form (k^2)+1 or (k^2)-1 that is divisible by N, and the guarantee that
>such a number exists depends on the theory of the Fermat-Pell 
>which no surviving manuscripts indicate was known by the ancients
>(although Diophantus and Archimedes may well have known this fact

I should clarify that the proof requires (k^2)+1 or (k^2)-1 to be a 
square times N, not just divisible by N. It shows that the square root 
of any integer k^2+1 or k^2-1 is irrational, from which one obtains the 
irrationality of any square root of an integer which can be multiplied 
by a square to get within 1 of a square.

Here is the solution to the historical mystery of why Theodorus stopped 
at 17:

n Size of triangle (relative to base) needed for proof
2  Side 1
3  Hypotenuse 2
5 Side 2
6 Hypotenuse 5
7 Hypotenuse 8
8 follows from 2
10 Side 3
11 Hypotenuse 10
12 follows from 3
13 Side 18
14 Hypotenuse 15
15 Hypotenuse 4
17 Side 4
18 follows from 2
19 Hypotenuse 170

-- JS

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