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## #26 2012-10-18 00:38:12

zetafunc.
Guest

### Re: Equation

So, generally you require a calculator to solve a Pell equation...?

## #27 2012-10-18 00:42:13

bobbym
bumpkin
From: Bumpkinland
Registered: 2009-04-12
Posts: 109,606

### Re: Equation

Depends on the Pell equation. For some of them like

you can tell just by looking at them. There is the method of Brahmagupta and Bhaskara two ancient Indian mathematicians.

Generally though, in computational number theory you will require the use of at least a good scientific calculator.

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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## #28 2012-10-18 01:01:56

zetafunc.
Guest

### Re: Equation

You can tell just by looking at them? You mean, finding the fundamental solution using their method, or by trial and error?

I found a useful recurrence relation which works for the OP's example. Unfortunately it does not help unless you have the fundamental solution.

## #29 2012-10-18 01:08:04

bobbym
bumpkin
From: Bumpkinland
Registered: 2009-04-12
Posts: 109,606

### Re: Equation

Yes, once you have the fundamental solution ( or any solution other than the trivial one ) you use a recurrence to get more solutions.

You can tell just by looking at them?

By inspection x = 24 and y = 5.

For that one you must compute the answer.

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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## #30 2012-10-18 02:24:44

zetafunc.
Guest

### Re: Equation

I must compute the answer for that one? I don't understand...

That Pell equation (the OP's original one) is even easier to solve by inspection, and once you have an (x,y) pair, it's easy to get the next solution (x = 199, y = 60). Why is it different to x^2 = 1 + 23y^2?

## #31 2012-10-18 02:31:42

bobbym
bumpkin
From: Bumpkinland
Registered: 2009-04-12
Posts: 109,606

### Re: Equation

When I say by inspection I mean without computing the convergents. The OP's one requires some computation. The other is as simple  as x +2 = 4.

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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## #32 2012-10-18 02:43:51

zetafunc.
Guest

### Re: Equation

I am able to get a solution without convergents...

## #33 2012-10-18 02:44:38

bobbym
bumpkin
From: Bumpkinland
Registered: 2009-04-12
Posts: 109,606

### Re: Equation

What did you get?

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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zetafunc.
Guest

x = 199, y = 60.

## #35 2012-10-18 02:49:06

bobbym
bumpkin
From: Bumpkinland
Registered: 2009-04-12
Posts: 109,606

### Re: Equation

You used the recurrence to get that. But if you want to get the fundamental solution convergents of the continued fraction is best.

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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## #36 2012-10-18 02:51:41

zetafunc.
Guest

### Re: Equation

But how can I compute them by hand? These problems often come to me when I have no computer or calculator near me, such as in an olympiad, or if someone comes up to me with a problem like it.

## #37 2012-10-18 02:57:10

bobbym
bumpkin
From: Bumpkinland
Registered: 2009-04-12
Posts: 109,606

### Re: Equation

They never come up in an olympiad. The only time someone asks that is when they know nothing and think the problem is easy. Guys like that think everything is as easy as plugging into the quadratic formula.

For instance, find the fundamental solution to:

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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## #38 2012-10-18 03:02:57

zetafunc.
Guest

### Re: Equation

How would Brahmagupta and Bhaskara approach that problem?

## #39 2012-10-18 03:08:51

bobbym
bumpkin
From: Bumpkinland
Registered: 2009-04-12
Posts: 109,606

### Re: Equation

They could not approach it. It is a problem that is historically famous. This is how the problem begins.

Archimedes wrote:

Page 1

If thou art diligent and wise, O stranger, compute the number of cattle of the Sun, who
once upon a time grazed on the ffields of the Thrinacian isle of Sicily, divided into four herds of
different colours, one milk white, another glossy black, the third yellow and the last dappled. In
each herd were bulls, mighty in number according to these proportions: Understand, stranger,
that the white bulls were equal to a half and a third of the black together with the whole of the
yellow, while the black were equal to the fourth part of the dappled and a fifth, together with,
once more, the whole of the yellow. Observe further that the remaining bulls, the dappled, were
equal to a sixth part of the white and a seventh, together with all the yellow. These were the
proportions of the cows: The white were precisely equal to the third part and a fourth of the
whole herd of the black; while the black were equal to the fourth part once more of the dappled
and with it a fifth part, when all, including the bulls, went to pasture together. Now the dappled
in four parts were equal in number to a fifth part and a sixth of the yellow herd. Finally the
yellow were in number equal to a sixth part and seventh of the white herd.
If thou canst accurately tell, O stranger, the number of Cattle of the Sun, giving separately
the number of well-fed bulls and again the number of females according to each colour, thou
wouldst not be called unskilled or ignorant of numbers, but not yet shalt thou be numbered
among the wise.
But come, understand also all these conditions regarding the cows of the Sun. When the
white bulls mingled their number with the black, they stood rm, equal in depth and breadth,
and the plains of Thrinacia, stretching far in all ways, were filled with their multitude. Again,
when the yellow and the dappled bulls were gathered into one herd they stood in such a manner
that their number, beginning from one, grew slowly greater till it completed a triangular figure,
there being no bulls of other colours in their midst nor one of them lacking.
If thou art able, O stranger, to find out all these things and gather them together in your
mind, giving all the relations, thou shalt depart crowned with glory and knowing that thou hast
been adjudged perfect in this species of wisdom.

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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## #40 2012-10-18 03:18:45

zetafunc.
Guest

### Re: Equation

How do you form the Pell equation from that?

I would not have liked to sit an exam set by Archimedes, that is for sure.

## #41 2012-10-18 03:23:55

bobbym
bumpkin
From: Bumpkinland
Registered: 2009-04-12
Posts: 109,606

### Re: Equation

The equation in post #37 is the Pell equation for that problem.

http://www.sciencenews.org/sn_arc98/4_1 … thland.htm

It took more that 1800 years for someone to partially solve the problem. It was only solved fully in the 20th century with the aid of computers.

Incidentally, the Pell equation is incorrectly named. Pell had nothing to do with it. It should be called Fermat's equation.

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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## #42 2012-10-18 03:30:32

zetafunc.
Guest

### Re: Equation

That is interesting, especially how they were only able to get the solution (all the digits) in 1965.

So, Archimedes probably knew that equations of that form were solvable.

## #43 2012-10-18 03:35:38

bobbym
bumpkin
From: Bumpkinland
Registered: 2009-04-12
Posts: 109,606

### Re: Equation

It would seem so. Vardi and others believe he at least formulated the problem correctly even if he was unable to solve for the 205 000 digit answer.

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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## #44 2012-10-19 01:34:04

Leroy
Guest

### Re: Equation

Thank you,now I can find a pell's equation's fundamental solution,but what is the method of finding the additional ones?please explain.

## #45 2012-10-19 01:38:50

bobbym
bumpkin
From: Bumpkinland
Registered: 2009-04-12
Posts: 109,606

### Re: Equation

You use two recurrences to find more answers.

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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## #46 2012-10-19 02:02:16

Leroy
Guest

### Re: Equation

You mean every 2nd convergent after the fundamental one?

## #47 2012-10-19 02:06:34

bobbym
bumpkin
From: Bumpkinland
Registered: 2009-04-12
Posts: 109,606

### Re: Equation

Hi;

Yes, that is one way if you have the convergents. x=numerator and y=denominator of the period.

There are other ways that do not require the convergents.

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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## #48 2012-10-19 02:29:07

Leroy
Guest

### Re: Equation

What ways could you please explain.(sorry for disturbing you so much)

## #49 2012-10-19 02:36:50

bobbym
bumpkin
From: Bumpkinland
Registered: 2009-04-12
Posts: 109,606

### Re: Equation

Hi Leroy;

No bother at all. You  remember that the fundamental solution is  x = 10 and y = 3. We use that to get all the rest.

With

Running the recurrences in the forward direction we get the first bunch:

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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## #50 2012-10-20 13:12:57

Leroy
Guest

### Re: Equation

Hi again,I have understood pell's equation,now I am curious about nagetive pell's equation,so is there any method to solve x^2-ny^2=-1