Cyclotomic field elements are not converted to Gap correctly

[saliola]

Although this works:

sage: K = CyclotomicField(3)
sage: z = K.an_element(); z
zeta3
sage: gap(z)
zeta3

the resulting gap element doesn't have the correct properties:

sage: K(gap.E(3)) == z  # Good!
True
sage: gap(K(gap.E(3))) == gap.E(3)  # Bad!
False

This causes the following problem with group characters.

sage: H = AlternatingGroup(4)
sage: g = H.list()[1]
sage: K = H.subgroup([g])
sage: z = CyclotomicField(3).an_element(); z
sage: c = K.character([1,z,z**2])
...
RuntimeError: Gap produced error output
Error, no 1st choice method found for `CONDUCTOR' on 1 arguments

Note: the above works if one takes z = gap.E(3).

CC: @wdjoyner

Component: algebra

Keywords: gap interface cyclotomic field

Author: Simon King

Reviewer: Luis Felipe Tabera Alonso

Merged: sage-4.6.2.alpha2

Issue created by migration from https://trac.sagemath.org/ticket/5618

[simon-king-jena]

comment:2

Here is some problem analysis:

sage: K = CyclotomicField(3)
sage: z = K.an_element(); z
zeta3
sage: gap(z)
zeta3
sage: K(gap(z))
zeta3
sage: gap.E(3)
E(3)
sage: gap.E(3) == gap(z)
False
sage: K(gap.E(3)) == K(gap(z)) == z
True

So, apparently GAP treats "the same" elements of a cyclotomic field differently if they have different names. But this isn't particularly surprising, since the two cyclotomic fields seem to have a totaly different representation in GAP:

sage: ZFgap = gap('CyclotomicField(3)')
sage: Kgap = gap(K)
sage: Kgap
<algebraic extension over the Rationals of degree 2>
sage: ZFgap
CF(3)
sage: ZFgap.GeneratorsOfField()
[ E(3) ]
sage: Kgap.GeneratorsOfField()
[ zeta3 ]

Note that comparison of the two field in the GAP interface results in an error:

sage: Kgap == ZFgap
ERROR: An unexpected error occurred while tokenizing input
The following traceback may be corrupted or invalid
The error message is: ('EOF in multi-line statement', (47, 0))

---------------------------------------------------------------------------
RuntimeError                              Traceback (most recent call last)

/home/king/<ipython console> in <module>()

/home/king/SAGE/sage-4.4.2/local/lib/python2.6/site-packages/sage/structure/element.so in sage.structure.element.Element.__richcmp__ (sage/structure/element.c:7061)()

/home/king/SAGE/sage-4.4.2/local/lib/python2.6/site-packages/sage/structure/element.so in sage.structure.element.Element._richcmp (sage/structure/element.c:6943)()

/home/king/SAGE/sage-4.4.2/local/lib/python2.6/site-packages/sage/interfaces/expect.pyc in __cmp__(self, other)
   1527                                  other.name())) == P._true_symbol():
   1528             return 0
-> 1529         elif P.eval("%s %s %s"%(self.name(), P._lessthan_symbol(), other.name())) == P._true_symbol():
   1530             return -1
   1531         elif P.eval("%s %s %s"%(self.name(), P._greaterthan_symbol(), other.name())) == P._true_symbol():

/home/king/SAGE/sage-4.4.2/local/lib/python2.6/site-packages/sage/interfaces/gap.pyc in eval(self, x, newlines, strip, **kwds)
    478             input_line += ';'
    479
--> 480         result = Expect.eval(self, input_line, **kwds)
    481
    482         if not newlines:

/home/king/SAGE/sage-4.4.2/local/lib/python2.6/site-packages/sage/interfaces/expect.pyc in eval(self, code, strip, synchronize, locals, **kwds)
    981         try:
    982             with gc_disabled():
--> 983                 return '\n'.join([self._eval_line(L, **kwds) for L in code.split('\n') if L != ''])
    984         except KeyboardInterrupt:
    985             # DO NOT CATCH KeyboardInterrupt, as it is being caught

/home/king/SAGE/sage-4.4.2/local/lib/python2.6/site-packages/sage/interfaces/gap.pyc in _eval_line(self, line, allow_use_file, wait_for_prompt)
    720                         return ''
    721                 else:
--> 722                     raise RuntimeError, message
    723
    724         except KeyboardInterrupt:

RuntimeError: Gap produced error output
Error, no 1st choice method found for `LT' on 2 arguments

   executing $sage2 < $sage8;

But GAP indeed considers the two fields to be different:

sage: gap.eval(Kgap.name() + ' = ' + ZFgap.name())
'false'

So, what does all this mean?

1. The __cmp__ method of the GAP interface has a bug. An error raised by GAP when attempting "<" or ">" should be caught and then "=" should be tried.
2. A GAP cyclotomic field is different from the GAP version of a Sage cyclotomic field. This is since the GAP version of a Sage cyclotomic field is a number field. It could be solved by providing a genuine _gap_init_ method for Sage cyclotomic fields (currently, it is inherited from number fields).

It seems likely to me that after implementing 2., things will already work. But 1. should be fixed as well.

[simon-king-jena]

Gap-representation of Cyclotomic Fields should be a Cyclotomic Field in Gap

[simon-king-jena]

Changed keywords from none to gap interface cyclotomic field

[simon-king-jena]

comment:3

Attachment: trac_5618_gap_for_cyclotomic_fields.patch.gz

I created the patch after the patches from #8909 and #9423 -- so, it is possible that the new patch actually depends on the two other tickets (but one of them already has a positive review).

With the patch, a cyclotomic field in Sage is represented as a cyclotomic field (and not as a number field) in GAP:

sage: Z = CyclotomicField(8)
sage: gap(Z)
CF(8)
sage: Z(gap(Z.0^2))
zeta8^2

The advantage is that the motivating example from the ticket description now works (and is used as a doctest):

sage: H = AlternatingGroup(4)
sage: g = H.list()[1]
sage: K = H.subgroup([g])
sage: z = CyclotomicField(3).an_element(); z
sage: c = K.character([1,z,z**2]); c
Character of Subgroup of AlternatingGroup(4) generated by [(2,3,4)]
sage: c(g^2); z^2
-zeta3 - 1
-zeta3 - 1

The disadvantage: While it is still possible to chose the name of a number field generator in GAP as in Sage

sage: K.<tau> = NumberField(x^2+x+1)
sage: gap(K)
<algebraic extension over the Rationals of degree 3>
sage: K.0
tau
sage: gap(K.0)
tau

it is now impossible to do the same for cyclotomic fields:

sage: L.<zeta> = CyclotomicField(3)
sage: L.0
zeta
sage: gap(L.0)
E(3)

By consequence, I had to fix a couple of doctests. All doctests pass, but I can not vouch for external code.

[simon-king-jena]

Author: Simon King

[lftabera]

comment:4

The patch simply converts sage Cyclotomic fields to gap cyclotomic fields instead of generic number fields. The disadvantage presented is just a limitation of gap, not sage.

The semantics of gap(CyclotomicField(n)) have changed!
DeprecationWarning would be too pedantic here. Current behaviour of sage is considered a bug and I cannot see any functionality loss. So the code is ok.

The doctests are relevant. However, this patch depends on #9423 and one doctest has disappeared in that patch. That doctest is relevant, because it shows the change in the code. So, I would add the output of:

sage: F=CyclotomicField(8)
sage: F.gen()
sage: F._gap_init_() # the following variable name $sage1 represents the F.base_ring() in gap and is somehow random
sage: f=gap(F)
sage: f.GeneratorsOfDivisionRing() 

in NumberField_cyclotomic.gap_init()

[lftabera]

Reviewer: Luis Felipe Tabera Alonso