How to solve this Pell's equation $x^{2} - 991y^{2} = 1 $

Solution 1:

If $x^2-991y^2=1$, then we have that $$ \begin{align} \left(\frac{1}{y}\right)^2 &=\left(\frac{x}{y}\right)^2-991\\ &=\left(\frac{x}{y}-\sqrt{991}\right)\left(\frac{x}{y}+\sqrt{991}\right)\tag{1} \end{align} $$ Since $\frac{x}{y}\stackrel{.}{=}\sqrt{991}\stackrel{.}{=}31.5$, we get that $$ \left|\frac{x}{y}-\sqrt{991}\right|\stackrel{.}{=}\frac1{63}\frac1{y^2}\tag{2} $$ As described in Section $5$ of this paper, to get a rational approximation as close as $(2)$, $\dfrac{x}{y}$ must be a convergent of the continued fraction for $\sqrt{991}$. Since the next continuant, c_n, satisfies $$ \frac1{c_n+2}\frac1{y^2}\lt\left|\frac{x}{y}-\sqrt{991}\right|\le\frac1{c_n}\frac1{y^2}\tag{3} $$ the next continuant should be within about $1$ of $62$.

The continued fraction for an algebraic number of degree $2$ will eventually repeat. The continued fraction for $\sqrt{991}$ is $$ (31, [2, 12, 10, 2, 2, 2, 1, 1, 2, 6, 1, 1, 1, 1, 3, 1, 8, 4, 1, 2, 1, 2, 3, 1, 4, 1, 20, 6, 4, 31, 4, 6, 20, 1, 4, 1, 3, 2, 1, 2, 1, 4, 8, 1, 3, 1, 1, 1, 1, 6, 2, 1, 1, 2, 2, 2, 10, 12, 2, 62]) $$ where the sequence in brackets repeats.

The only continuant within $1$ of $62$ is the $62$. Thus, the first positive solution corresponds to the rational approximation to $\sqrt{991}$ with the continued fraction $$ (31, 2, 12, 10, 2, 2, 2, 1, 1, 2, 6, 1, 1, 1, 1, 3, 1, 8, 4, 1, 2, 1, 2, 3, 1, 4, 1, 20, 6, 4, 31, 4, 6, 20, 1, 4, 1, 3, 2, 1, 2, 1, 4, 8, 1, 3, 1, 1, 1, 1, 6, 2, 1, 1, 2, 2, 2, 10, 12, 2) $$ which is $$ \color{#C00000}{\frac{x_1}{y_1}}=\color{#C00000}{\frac{379516400906811930638014896080}{12055735790331359447442538767}}\tag{4} $$ The next solution comes from the next time the next continuant is $62$; that is, for the rational approximation to $\sqrt{991}$ with the continued fraction $$ (31, 2, 12, 10, 2, 2, 2, 1, 1, 2, 6, 1, 1, 1, 1, 3, 1, 8, 4, 1, 2, 1, 2, 3, 1, 4, 1, 20, 6, 4, 31, 4, 6, 20, 1, 4, 1, 3, 2, 1, 2, 1, 4, 8, 1, 3, 1, 1, 1, 1, 6, 2, 1, 1, 2, 2, 2, 10, 12, 2, 62, 2, 12, 10, 2, 2, 2, 1, 1, 2, 6, 1, 1, 1, 1, 3, 1, 8, 4, 1, 2, 1, 2, 3, 1, 4, 1, 20, 6, 4, 31, 4, 6, 20, 1, 4, 1, 3, 2, 1, 2, 1, 4, 8, 1, 3, 1, 1, 1, 1, 6, 2, 1, 1, 2, 2, 2, 10, 12, 2) $$ which is $$ \color{#C00000}{\frac{x_2}{y_2}}=\color{#C00000}{\frac{288065397114519999215772221121510725946342952839946398732799}{9150698914859994783783151874415159820056535806397752666720}}\tag{5} $$ We can get all the solutions, starting with $(x_0,y_0)=(1,0)$ and $(4)$, using $$ \begin{align} x_n&=759032801813623861276029792160\,x_{n-1}-x_{n-2}\\ y_n&=759032801813623861276029792160\,y_{n-1}-y_{n-2} \end{align}\tag{6} $$

Solution 2:

Edit, 6 April 2014: I put some GMP oversize integers in two of my indefinite forms programs in C++, so here is the Lagrange cycle method for doing this. The importance of this is that no decimal accuracy is required at all, and no pattern recognition (cycle detection??), no memory is required at all.

Pell 991

0  form   1 62 -30   delta  -2
1  form   -30 58 5   delta  12
2  form   5 62 -6   delta  -10
3  form   -6 58 25   delta  2
4  form   25 42 -22   delta  -2
5  form   -22 46 21   delta  2
6  form   21 38 -30   delta  -1
7  form   -30 22 29   delta  1
8  form   29 36 -23   delta  -2
9  form   -23 56 9   delta  6
10  form   9 52 -35   delta  -1
11  form   -35 18 26   delta  1
12  form   26 34 -27   delta  -1
13  form   -27 20 33   delta  1
14  form   33 46 -14   delta  -3
15  form   -14 38 45   delta  1
16  form   45 52 -7   delta  -8
17  form   -7 60 13   delta  4
18  form   13 44 -39   delta  -1
19  form   -39 34 18   delta  2
20  form   18 38 -35   delta  -1
21  form   -35 32 21   delta  2
22  form   21 52 -15   delta  -3
23  form   -15 38 42   delta  1
24  form   42 46 -11   delta  -4
25  form   -11 42 50   delta  1
26  form   50 58 -3   delta  -20
27  form   -3 62 10   delta  6
28  form   10 58 -15   delta  -4
29  form   -15 62 2   delta  31
30  form   2 62 -15   delta  -4
31  form   -15 58 10   delta  6
32  form   10 62 -3   delta  -20
33  form   -3 58 50   delta  1
34  form   50 42 -11   delta  -4
35  form   -11 46 42   delta  1
36  form   42 38 -15   delta  -3
37  form   -15 52 21   delta  2
38  form   21 32 -35   delta  -1
39  form   -35 38 18   delta  2
40  form   18 34 -39   delta  -1
41  form   -39 44 13   delta  4
42  form   13 60 -7   delta  -8
43  form   -7 52 45   delta  1
44  form   45 38 -14   delta  -3
45  form   -14 46 33   delta  1
46  form   33 20 -27   delta  -1
47  form   -27 34 26   delta  1
48  form   26 18 -35   delta  -1
49  form   -35 52 9   delta  6
50  form   9 56 -23   delta  -2
51  form   -23 36 29   delta  1
52  form   29 22 -30   delta  -1
53  form   -30 38 21   delta  2
54  form   21 46 -22   delta  -2
55  form   -22 42 25   delta  2
56  form   25 58 -6   delta  -10
57  form   -6 62 5   delta  12
58  form   5 58 -30   delta  -2
59  form   -30 62 1   delta  62
60  form   1 62 -30

 disc   3964
Automorph, written on right of Gram matrix:  
5788591406539787767296194303  361672073709940783423276163010
12055735790331359447442538767  753244210407084073508733597857


 Pell automorph 
379516400906811930638014896080  11947234168218377212415555918097
12055735790331359447442538767  379516400906811930638014896080

Pell unit 
379516400906811930638014896080^2 - 991 * 12055735790331359447442538767^2 = 1 

=========================================

991       991

I give a complete description at How to detect when continued fractions period terminates and at the MO question I link to there. You need to be very confident about using two by two matrices. However, and this is the key point, it is NOT NECESSARY to use high decimal precision. All is integer arithmetic. The only approximation is the integer part of the square root of the discriminant, as in $\left\lfloor \sqrt {3964} \right\rfloor = 62.$ The bad news is that my program is limited to number s below $2^{31},$ so the "answer" at the end was gibberish and I just erased it. Oh, this comes up sometimes...with this method, it is also not necessary to use any cycle detection. The cycle for $991$ begins with the binary form $\langle 1,62,-30 \rangle$ and ends precisely when we, once again, reach $\langle 1,62,-30 \rangle.$ Not before, not after. I have a short example, the triples may repeat the first entry, in this case 9, but you are done when all three entries match:

0  form   9 75 -16   delta  -4
1  form   -16 53 53   delta  1
2  form   53 53 -16   delta  -4
3  form   -16 75 9   delta  8
4  form   9 69 -40   delta  -1
5  form   -40 11 38   delta  1
6  form   38 65 -13   delta  -5
7  form   -13 65 38   delta  1
8  form   38 11 -40   delta  -1
9  form   -40 69 9   delta  8
10  form   9 75 -16

Let me start with an easier one, 91, where I do get an answer, then show 991 after:

jagy@phobeusjunior:~/old drive/home/jagy/Cplusplus$ ./Pell
Input n for Pell 
91

0  form   1 18 -10   delta  -1
1  form   -10 2 9   delta  1
2  form   9 16 -3   delta  -5
3  form   -3 14 14   delta  1
4  form   14 14 -3   delta  -5
5  form   -3 16 9   delta  1
6  form   9 2 -10   delta  -1
7  form   -10 18 1   delta  18
8  form   1 18 -10

 disc   364
Automorph, written on right of Gram matrix:  
89  1650
165  3059


 Pell automorph 
1574  15015
165  1574

Pell unit 
1574^2 - 91 * 165^2 = 1 

=========================================
jagy@phobeusjunior:~/old drive/home/jagy/Cplusplus$ 



jagy@phobeusjunior:~/old drive/home/jagy/Cplusplus$ 
jagy@phobeusjunior:~/old drive/home/jagy/Cplusplus$ ./indefCycle
Input three coefficients a b c for indef f(x,y)= a x^2 + b x y + c y^2 
1 0 -991

  0  form              1           0        -991  delta      0
  1  form           -991           0           1  delta     31
  2  form              1          62         -30


          -1         -31
           0          -1

To Return  
          -1          31
           0          -1

0  form   1 62 -30   delta  -2
1  form   -30 58 5   delta  12
2  form   5 62 -6   delta  -10
3  form   -6 58 25   delta  2
4  form   25 42 -22   delta  -2
5  form   -22 46 21   delta  2
6  form   21 38 -30   delta  -1
7  form   -30 22 29   delta  1
8  form   29 36 -23   delta  -2
9  form   -23 56 9   delta  6
10  form   9 52 -35   delta  -1
11  form   -35 18 26   delta  1
12  form   26 34 -27   delta  -1
13  form   -27 20 33   delta  1
14  form   33 46 -14   delta  -3
15  form   -14 38 45   delta  1
16  form   45 52 -7   delta  -8
17  form   -7 60 13   delta  4
18  form   13 44 -39   delta  -1
19  form   -39 34 18   delta  2
20  form   18 38 -35   delta  -1
21  form   -35 32 21   delta  2
22  form   21 52 -15   delta  -3
23  form   -15 38 42   delta  1
24  form   42 46 -11   delta  -4
25  form   -11 42 50   delta  1
26  form   50 58 -3   delta  -20
27  form   -3 62 10   delta  6
28  form   10 58 -15   delta  -4
29  form   -15 62 2   delta  31
30  form   2 62 -15   delta  -4
31  form   -15 58 10   delta  6
32  form   10 62 -3   delta  -20
33  form   -3 58 50   delta  1
34  form   50 42 -11   delta  -4
35  form   -11 46 42   delta  1
36  form   42 38 -15   delta  -3
37  form   -15 52 21   delta  2
38  form   21 32 -35   delta  -1
39  form   -35 38 18   delta  2
40  form   18 34 -39   delta  -1
41  form   -39 44 13   delta  4
42  form   13 60 -7   delta  -8
43  form   -7 52 45   delta  1
44  form   45 38 -14   delta  -3
45  form   -14 46 33   delta  1
46  form   33 20 -27   delta  -1
47  form   -27 34 26   delta  1
48  form   26 18 -35   delta  -1
49  form   -35 52 9   delta  6
50  form   9 56 -23   delta  -2
51  form   -23 36 29   delta  1
52  form   29 22 -30   delta  -1
53  form   -30 38 21   delta  2
54  form   21 46 -22   delta  -2
55  form   -22 42 25   delta  2
56  form   25 58 -6   delta  -10
57  form   -6 62 5   delta  12
58  form   5 58 -30   delta  -2
59  form   -30 62 1   delta  62
60  form   1 62 -30
=========================================
jagy@phobeusjunior:~/old drive/home/jagy/Cplusplus$

Solution 3:

Following the link, taking (379,12, which was calculated by brute force) as the fundamental solution, we can get the generic solution.

I'm trying to replace the brute force with human intelligence(!) using the followings: first , second and continued fraction of $\sqrt{991}$

Solution 4:

Here are the steps to find the solutions for all d = m^2 + 2k. In your question d = 991 which belongs to this category. The following five steps give the first solution and the fundamental solutions.

1) 991 = m^2 + 2k

991 - 31^2 = 2k

So m=31, k=15.

2) We set x = m^2 - m + k = 945

y = m - 1 = 30

945^2 - 991*30^2 = 1125 -> 189 - 991*6^2 = 45 -> 63^2 - 991*2^2 = 5

3) Now we will find the solvent as follows:

x_1 = m^2 + m + 2k = 1022

y_1 = m + 1 = 32

1022^2 - 991*32^2 = 29700 -> 511^2 - 991*16^2 = 7425

x_2 = m^2 - m + 2k = 960

y_2 = m - 1 = 30