# set precision for pari in Sage

In the pari program, we can use \pb 256 to set the precision. For the pari in Sage, how to use this command ?

1

To change real precision in gp run `gp.default("realprecision",256)`

.

However, this issue in your code is not in pari's precision per se, but in loss of precision when pari's object is converted to Sage (it may be a bug in `cypari2`

). Here is an equivalent code with explicit precision control (for both Sage and Pari) based on `gp.`

interface:

```
myC = ComplexField(200) # precision in bits, we use it for conversion to Sage
gp.default("realprecision",60) # precision in decimal digits for pari's routines
K.<a>=CyclotomicField(3)
E=gp.ellinit([0,0,0,0,-432*(1+3*a)^4])
S=gp.mfinit([63,2,Mod(37,63)])
L=gp.mfeigenbasis(S)
symb = gp.mfsymbol(L[1])
def phiE(c):
return myC(gp.mfsymboleval(symb,[gp.oo(), c]))*2*myC(pi)*I
print(phiE(gp.oo()))
print(phiE(0))
print(phiE(1/1000))
```

1

maybe

```
pari.set_real_precision(prec)
```

Get its documentation with `?`

:

```
sage: pari.set_real_precision?
Signature: pari.set_real_precision(n)
Docstring:
Sets the PARI default real precision in decimal digits.
This is used both for creation of new objects from strings and for
printing. It is the number of digits *IN DECIMAL* in which real
numbers are printed. It also determines the precision of objects
created by parsing strings (e.g. pari('1.2')), which is *not* the
normal way of creating new PARI objects in CyPari2. It has *no*
effect on the precision of computations within the pari library.
Returns the previous PARI real precision.
See also: "set_real_precision_bits()" to set the precision in bits.
Examples:
>>> import cypari2
>>> pari = cypari2.Pari()
>>> pari.set_real_precision(60)
15
>>> pari('1.2')
1.20000000000000000000000000000000000000000000000000000000000
>>> pari.set_real_precision(15)
60
Init docstring: Initialize self. See help(type(self)) for accurate signature.
File:
Type: method
```

Type `pari.set`

and press the `TAB`

key for a list of similar functions.

```
sage: pari.set
pari.set_debug_level
pari.set_real_precision
pari.set_real_precision_bits
pari.set_series_precision
pari.setbinop
pari.setintersect
pari.setisset
pari.setminus
pari.setrand
pari.setsearch
pari.setunion
```

For example, I run the following code in sage

```
from cypari2 import Pari
pari = Pari()
print(R)
K.<a>=CyclotomicField(3)
E=pari([0,0,0,0,-432*(1+3*a.n(3000))^4]).ellinit()
S=pari([63,2,Mod(37,63)]).mfinit()
L=pari(S).mfeigenbasis()
symb=L[0].mfsymbol();
def phiE(c):
#return E.ellztopoint(pari(symb.mfsymboleval([oo, c])*2*pi*I).polcoef(0))
return symb.mfsymboleval([oo, c])*2*pi*I
#return pari(symb.mfsymboleval([oo, c])*2*pi*I).polcoef(0)
print(phiE(oo))
print(phiE(0))
print(phiE(1/1000))
```

the output is with very little precision. How can I get the output with high precision?

This is not an issue with precision in pari but rather a loss of precision when pari object `symb.mfsymboleval([oo, c])`

is converted to Sage. Perhaps, it's a bug in cypari2.

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Asked: ** 2022-03-23 08:37:44 +0200 **

Seen: **267 times**

Last updated: **Mar 26 '22**

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