Interface between Sage and PARI

Guide to real precision in the PARI interface

In the PARI interface, “real precision” refers to the precision of real numbers, so it is the floating-point precision. This is a non-trivial issue, since there are various interfaces for different things.

Internal representation and conversion between Sage and PARI

Real numbers in PARI have a precision associated to them, which is always a multiple of the CPU wordsize. So, it is a multiple of 32 of 64 bits. When converting from Sage to PARI, the precision is rounded up to the nearest multiple of the wordsize:

sage: x = 1.0
sage: x.precision()
53
sage: pari(x)
1.00000000000000
sage: pari(x).bitprecision()
64

With a higher precision:

sage: x = RealField(100).pi()
sage: x.precision()
100
sage: pari(x).bitprecision()
128

When converting back to Sage, the precision from PARI is taken:

sage: x = RealField(100).pi()
sage: y = pari(x).sage()
sage: y
3.1415926535897932384626433832793333156
sage: parent(y)
Real Field with 128 bits of precision

So pari(x).sage() is definitely not equal to x since it has 28 bogus bits.

Therefore, some care must be taken when juggling reals back and forth between Sage and PARI. The correct way of avoiding this is to convert pari(x).sage() back into a domain with the right precision. This has to be done by the user (or by Sage functions that use PARI library functions). For instance, if we want to use the PARI library to compute sqrt(pi) with a precision of 100 bits:

sage: R = RealField(100)
sage: s = R(pi); s
3.1415926535897932384626433833
sage: p = pari(s).sqrt()
sage: x = p.sage(); x    # wow, more digits than I expected!
1.7724538509055160272981674833410973484
sage: x.prec()           # has precision 'improved' from 100 to 128?
128
sage: x == RealField(128)(pi).sqrt()  # sadly, no!
False
sage: R(x)               # x should be brought back to precision 100
1.7724538509055160272981674833
sage: R(x) == s.sqrt()
True

Output precision for printing

Even though PARI reals have a precision, not all significant bits are printed by default. The maximum number of digits when printing a PARI real can be set using the methods Pari.set_real_precision_bits() or Pari.set_real_precision().

We create a very precise approximation of pi and see how it is printed in PARI:

sage: pi = pari(RealField(1000).pi())

The default precision is 15 digits:

sage: pi
3.14159265358979

With a different precision:

sage: _ = pari.set_real_precision(50)
sage: pi
3.1415926535897932384626433832795028841971693993751

Back to the default:

sage: _ = pari.set_real_precision(15)
sage: pi
3.14159265358979

Input precision for function calls

When we talk about precision for PARI functions, we need to distinguish three kinds of calls:

  1. Using the string interface, for example pari("sin(1)").
  2. Using the library interface with exact inputs, for example pari(1).sin().
  3. Using the library interface with inexact inputs, for example pari(1.0).sin().

In the first case, the relevant precision is the one set by the methods Pari.set_real_precision_bits() or Pari.set_real_precision():

sage: pari.set_real_precision_bits(150)
sage: pari("sin(1)")
0.841470984807896506652502321630298999622563061
sage: pari.set_real_precision_bits(53)
sage: pari("sin(1)")
0.841470984807897

In the second case, the precision can be given as the argument precision in the function call, with a default of 53 bits. The real precision set by Pari.set_real_precision_bits() or Pari.set_real_precision() is irrelevant.

In these examples, we convert to Sage to ensure that PARI’s real precision is not used when printing the numbers. As explained before, this artificially increases the precision to a multiple of the wordsize.

sage: s = pari(1).sin(precision=180).sage(); print(s); print(parent(s))
0.841470984807896506652502321630298999622563060798371065673
Real Field with 192 bits of precision
sage: s = pari(1).sin(precision=40).sage(); print(s); print(parent(s))
0.841470984807896507
Real Field with 64 bits of precision
sage: s = pari(1).sin().sage(); print(s); print(parent(s))
0.841470984807896507
Real Field with 64 bits of precision

In the third case, the precision is determined only by the inexact inputs and the precision argument is ignored:

sage: pari(1.0).sin(precision=180).sage()
0.841470984807896507
sage: pari(1.0).sin(precision=40).sage()
0.841470984807896507
sage: pari(RealField(100).one()).sin().sage()
0.84147098480789650665250232163029899962

Elliptic curve functions

An elliptic curve given with exact \(a\)-invariants is considered an exact object. Therefore, you should set the precision for each method call individually:

sage: e = pari([0,0,0,-82,0]).ellinit()
sage: eta1 = e.elleta(precision=100)[0]
sage: eta1.sage()
3.6054636014326520859158205642077267748
sage: eta1 = e.elleta(precision=180)[0]
sage: eta1.sage()
3.60546360143265208591582056420772677481026899659802474544