Bill Allombert on Tue, 8 Oct 2002 16:11:05 +0200

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 Re: precision and contfrac()

```On Tue, Oct 08, 2002 at 02:43:50PM +0200, Thomas Baruchel wrote:
> Brest, le mardi 8 octobre
>
> Hi, I wonder how I should set the precision \p in order to
> have n exact terms in the continued fraction expansion of
> sqrt(x) (x being exact).

It is a difficult question. The precision exhausted depend on the size of the
partial quotients, which are very hard to control.

>
> For instance, what precision is needed in order to be sure that I
> have 500 exact terms ? 2,000 ? 10,000 ?

You do not need to use reals for this computation:

Try this script, similar to examples/contrac.gp

cf(D,n) output the n first partial quotient of sqrt(D),
without relying on real numbers.

cf(D,n) =
{
local(a,r,u,v);

if (type(D) != "t_INT" || D < 2, return(-1));
u = sqrtint(D); v = D-u^2;
if (!v, return(0));
r = u;
vector(n,i,
if(i==1,u,a=(u+r)\v;
u = (r+u)\v * v - u;
v = (D-u^2)\v;a
));
}

Cheers,
Bill.
```