## Sunday, October 11, 2020

### Casio Classpad: Theorem of Sós

Introduction: The Theorem of Sós

A set of triple rational numbers (a, b, λ) is considered to be a H-triple if

c = √(a^2 - 2 λ a b + b^2) is also a rational number.  (I)

Heron Triple

Should the set of rational numbers (a, b, λ) can be further test to see if to is also a Heron triple.  That criteria (see source) requires for some numbers λ' and A (area):

λ' = √(1 - λ^2) is a positive rational number, and

A = λ' * abs(a * b) / 2 is a natural number  (positive integer)

This implies for all cases for Heron Triples that abs(λ) < 1, or -1 < λ < 1.

If the H-triple passes, then it becomes a Heron triple and the sides (a, b, c) (see (I) above) are the lengths of a Heron triangle.

It is possible that an H-triple is not a Heron triple.

Today's blog focuses on H-triples.

Generating H-Triples

Given relatively prime integers m and n (where gcd(m,n) = 1) and μ, an H-triple can be generated by the following equations:

(II)

a = μ * (m^2 - n^2)

b = μ * (2*m * (n + λ*m))

c = μ * (m^2 + 2*λ*m*n + n^2)

For a > 0, this requires that m > n.  In the program htrigen, I assume that μ = 1.  In the program, u = λ

(fx-CP400 and fx-CG500)

'2020-09-15 EWS

'H Triple

Local a,b,c,m,n,u

Input m, "m > n", "m?"

Input n, "m > n", "n?"

Input u, "λ? (rational)"

If gcd(m,n)≠1

Then

Print "m and n are not"

Print "relatively prime."

Stop

IfEnd

(m^2 - n^2) ⇒ a

(2 × m × (n + u × m)) ⇒ b

(m^2 + 2 × u × m × n + n^2) ⇒ c

Print "H-Triple:", ColorMagenta

Print "{a,b,λ}="

Print {a,b,u}, ColorBlue

Print "c="

Print c

Example:

m = 7, n = 3, λ = 2/3

Result:

a = 40, b = 322/3, c = 86

Testing the Theorem of Sós

The program htrist tests the criteria for the triple (a, b, λ).

Testing numbers in a program to see that they are rational is not as easy as it appears. Thankfully, the functions numerator and denominator can be used on the Classpad.  The numerator function converts the number into an exact fraction and extracts its numerator.  Similarly, the denominator function converts the number into an exact fraction and extracts its denominator.

For an irrational number such as √2, the Classpad determines the exact fraction to be

√2/1.

(fx-CP400 and fx-CG500)

'2020-09-15 EWS

'H Triple test

Local a,b,c,n,d,u

Input a

Input b

Input u, "λ? (rational)"

√(a^2 - 2 × u × a × b + b^2) ⇒ c

numerator(c) ⇒ n

denominator(c) ⇒ d

ClrText

Print {n, d}

If frac(n) = 0 and frac(d) = 0

Then

Print "H Triple", ColorGreen

Print {a, b, u}

Print "c ="

Print c

Else

Print "No Solution", ColorRed

IfEnd

Example:

a = 85, b = 374, λ = 1

Results:

c = 289,  n = 289, d = 1

Source:

Halbeisen, Lorezn and Hungrebühler, Nobert.  "Heron triangles and their elliptic curves" Journal of Number Theory 213 (2020) 232-253.  https://doi.org/10.1016/j.jnt.2019.12.005

Eddie

All original content copyright, © 2011-2020.  Edward Shore.   Unauthorized use and/or unauthorized distribution for commercial purposes without express and written permission from the author is strictly prohibited.  This blog entry may be distributed for noncommercial purposes, provided that full credit is given to the author.