## [sub-project] Perfect cuboid

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x3mEn
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### Re: [sub-project] Perfect cuboid

Zak, no problem, buу Wolfram|Alpha Pro to get the proof.

x3mEn
XBOX360-Installer Beiträge: 99
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### Re: [sub-project] Perfect cuboid

Some ideas regarding the last equation.

As far as
• One edge, two face diagonals and the body diagonal must be odd, one edge and the remaining face diagonal must be divisible by 4, and the remaining edge must be divisible by 16.
• Two edges must have length divisible by 3 and at least one of those edges must have length divisible by 9.
• One edge must have length divisible by 5.
• One edge must have length divisible by 7.
• One edge must have length divisible by 11.
• One edge must have length divisible by 19.
• One edge or space diagonal must be divisible by 13.
• The space diagonal can only contain prime divisors ≡ 1(mod 4) CodeCogs4.gif (13.29 KiB) 3452 mal betrachtet
When we know a factorization of y, we can combine all possible values of d^2+e^2+f^2 as a product of 2 and primes 4k+1 in some powers, and check further equations.

Zak
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### Re: [sub-project] Perfect cuboid

x3Men
If I havn't mistake: d.gif (40.67 KiB) 3434 mal betrachtet
PS
Please draw your attention to the sign +/-. Face diagonal d must be integer for both cases.

x3mEn
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Registriert: 20.03.2011 22:23

### Re: [sub-project] Perfect cuboid

Zak, you are muddling up 'necessary' and 'enough'.
If undersquare expression is 2f^2e^2, yes, it's enough to aquire z as a full square of the sum or the difference of f^2 and e^2.
But, is it necessary? No.
There are a lot of other expressions binding f and e which are a full square too.
The world is not limited only by (f^2+e^2)^2 and (f^2—e^2)^2 expressions.
And, btw, undersquare expression must be not only a full square, but a power of 4.

Zak
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### Re: [sub-project] Perfect cuboid

x3mEn
O.K. but if there are a lot of other solutions, why we can't to find at least one of them until at least 2^53 after all, the fourh exponent is not too big?

x3mEn
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### Re: [sub-project] Perfect cuboid

Zak hat geschrieben:the fourh exponent is not too big?
It's time to talk about how Math works. With the help of strict math logic of equivalent expressions you transfered from the given system of 4 Diophantine equations to consequent (and, btw, not necessary equivalent to input system) expression, which says: "If the given system of equations has a solution over integers, then the consequent expression is true over those values of system solutions.
The complexity of your consequent expression, even if it contains numbers in 8 degrees, says nothing about the complexity of calculations which can (should) lead to desired solution.
The fourth exponent is needed only to verify expressed by you equation.
Zak hat geschrieben:O.K. but if there are a lot of other solutions, why we can't to find at least one of them until at least 2^53 after all
That's why the Perfect cuboid problems thousands of years still stays on top of unsolved Math problems! 3 systems of 3 Diophantine equations have lots (and I believe, infinite) of solutions: CodeCogsEqn.gif (2.68 KiB) 3384 mal betrachtet
But the system of 4 Diophantine equations stays unsolved and nobody knows that's because it hasn't solutions or it has but the numbers are huge?

For instance, the Diophantine equation (beneath) has solutions, but known numbers are very huge! And nobody knows why. CodeCogsXYZ.gif (724 Bytes) 3384 mal betrachtet
Here is the simplest known solution:
x = 154476802108746166441951315019919837485664325669565431700026634898253202035277999
y = 36875131794129999827197811565225474825492979968971970996283137471637224634055579
z = 4373612677928697257861252602371390152816537558161613618621437993378423467772036

Zak
Prozessor-Polier Beiträge: 109
Registriert: 04.11.2017 12:27

### Re: [sub-project] Perfect cuboid

x3mEn hat geschrieben: With the help of strict math logic of equivalent expressions you transfered from the given system of 4 Diophantine equations to consequent (and, btw, not necessary equivalent to input system) expression, which says: "If the given system of equations has a solution over integers, then the consequent expression is true over those values of system solutions.
Not only. Decision of equation: defx.JPG (13.54 KiB) 3374 mal betrachtet
in integers will be equal to full task, because if we will have d,e,f, then we can find all the rest. We can find g and then we can find a,b,c.

x3mEn
XBOX360-Installer Beiträge: 99
Registriert: 20.03.2011 22:23

### Re: [sub-project] Perfect cuboid

Zak hat geschrieben:Decision of equation in integers will be equal to full task, because if we will have d,e,f, then we can find all the rest. We can find g and then we can find a,b,c.
You are not right.
When you have a system of Diophantine X equations with Y variables, theoretically you can simplify it to an equation of Y-X+1 variables, but only to prove of non-existance of solutions. But you cannot use this one simplified equation of Y-X+1 variables to solve the whole system. For example: CodeCogsEqn1.gif (7.64 KiB) 3365 mal betrachtet
So, be careful with a system of Diophantine equations in terms of equivalence.

Zak
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### Re: [sub-project] Perfect cuboid

x3mEn
I know about you had showed. But I told about other equation whith new integer variable x, which included sqr(16a^2b^2c^2g^2). The equation will be TRUE only in case when d,e,f will correspond perfect brick's face diagonals.
By means of transformations and finding the roots of the equation, we simply excluded cases that are not of interest to us.

Zak
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Registriert: 04.11.2017 12:27

### Re: [sub-project] Perfect cuboid

The problem is to find such integer values of x, f, e for which d would be integer in both cases. This is not possible in my humble opinion. ic.JPG (22.56 KiB) 3329 mal betrachtet
Geometric mean of squares of two integers gives the square of an integer.

Zak
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### Re: [sub-project] Perfect cuboid

x3mEn ic2.JPG (21.43 KiB) 3329 mal betrachtet
I even believe that, in principle, this case is not possible too.
It is generally not possible for both roots to get integer values.

x3mEn
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Registriert: 20.03.2011 22:23

### Re: [sub-project] Perfect cuboid

Zak hat geschrieben:x3mEn
I know about you had showed. But I told about other equation whith new integer variable x, which included sqr(16a^2b^2c^2g^2). The equation will be TRUE only in case when d,e,f will correspond perfect brick's face diagonals.
By means of transformations and finding the roots of the equation, we simply excluded cases that are not of interest to us.
If you will find integer d,e,f,x and calculate a,b,c,g based on d,e,f, they (I mean a,b,c,g) will not necessarily be integer, some of them or even entire all can turn to irrational. Despite the fact that x^2=16a^2*b^2*c^2*g^2 will be TRUE.
Because a product of irrational numbers could be an integer, and not just an integer, but a full square. Easy.