Opened 11 years ago

Closed 10 years ago

# `quo_rem` for divisor of leading unit coefficient

Reported by: Owned by: Kwankyu Lee Alex Ghitza minor sage-5.8 basic arithmetic fix and doctest exceptions sage-5.8.beta2 Kwankyu Lee Frédéric Chapoton N/A #12612

### Description

`quo_rem` does not work for divisors of unit leading coefficients, though the quotient and the remainder are defined mathematically.

```sage: R.<y> = QQ[x][]
sage: f = R.random_element(200)
sage: g = y^100+R.random_element(99)
sage: q, r = f.quo_rem(g)
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
...
AttributeError: 'sage.rings.polynomial.polynomial_element.Polynomial_generic_dense' object has no attribute 'quo_rem'
```

### comment:1 Changed 11 years ago by Kwankyu Lee

Milestone: sage-4.7.2 → sage-4.7.1 new → needs_review

### comment:2 Changed 11 years ago by Kwankyu Lee

Authors: → Kwankyu Lee

### comment:3 Changed 11 years ago by Kwankyu Lee

The patch allows one to compute the quotient and the remainder for divisors of unit leading coefficient. The `quo_rem` added to `Polynomial_generic_dense` is slightly faster than `quo_rem` defined in `Polynomial_generic_field`.

### comment:4 Changed 11 years ago by John Cremona

Status: needs_review → needs_info

In the case where self is 0 should it not be self, other returned? Not self, self?

### comment:5 follow-up:  6 Changed 11 years ago by Kwankyu Lee

The quotient and the remainder of 0(=self) divided by other is 0 and 0. So self, self is correct.

By the way, I did not care for this patch for a while. It needs to be rebased for the current Sage beta. I can't do it right away. I would appreciate if you do it.

### comment:6 in reply to:  5 ; follow-up:  7 Changed 11 years ago by John Cremona

Status: needs_info → needs_review

The quotient and the remainder of 0(=self) divided by other is 0 and 0. So self, self is correct.

OK, I was being stupid.

By the way, I did not care for this patch for a while. It needs to be rebased for the current Sage beta. I can't do it right away. I would appreciate if you do it.

Well, I have many other things to do -- especially as at present I am running Sage Days 35.

### comment:7 in reply to:  6 Changed 11 years ago by Kwankyu Lee

Well, I have many other things to do -- especially as at present I am running Sage Days 35.

Sorry for the suggestion. You are now the busiest man in the Sage community. :-) Good wish for Sage Days 35.

### comment:8 Changed 11 years ago by Kwankyu Lee

I see no doctest failures on my own Ubuntu 11.10 and Mac. I don't know what to do with the patchbot results.

### comment:9 Changed 11 years ago by Kwankyu Lee

Removing spaces does not help. :-(

### comment:10 follow-up:  11 Changed 11 years ago by aapitzsch

Looking forward to Python3, use the "new" syntax to raise errors, i.e.

```raise ZeroDivisionError("division by zero polynomial")
```

### comment:11 in reply to:  10 Changed 11 years ago by Kwankyu Lee

Looking forward to Python3, use the "new" syntax to raise errors, i.e.

```raise ZeroDivisionError("division by zero polynomial")
```

Done.

### comment:12 Changed 11 years ago by David Loeffler

Status: needs_review → needs_work → fails doctests

According to the patchbot, some doctests are fail with this patch on the current beta (5.0.beta8):

```sage -t  -force_lib devel/sage-11593/sage/rings/function_field/function_field_element.pyx
**********************************************************************
File "/storage/masiao/sage-5.0.beta8-patchbot/devel/sage-11593/sage/rings/function_field/function_field_element.pyx", line 423:
sage: a = ~(2*y + 1/x); a                           # indirect doctest
Expected:
(-x^2/(8*x^5 + x^2 + 1/2))*y + (2*x^3 + x)/(16*x^5 + 2*x^2 + 1)
Got:
(-1/2*x^2/(4*x^5 + 1/2*x^2 + 1/4))*y + (1/2*x^3 + 1/4*x)/(4*x^5 + 1/2*x^2 + 1/4)
**********************************************************************
File "/storage/masiao/sage-5.0.beta8-patchbot/devel/sage-11593/sage/rings/function_field/function_field_element.pyx", line 443:
sage: a = ~(2*y + 1/x); a
Expected:
(-x^2/(8*x^5 + x^2 + 1/2))*y + (2*x^3 + x)/(16*x^5 + 2*x^2 + 1)
Got:
(-1/2*x^2/(4*x^5 + 1/2*x^2 + 1/4))*y + (1/2*x^3 + 1/4*x)/(4*x^5 + 1/2*x^2 + 1/4)
**********************************************************************
File "/storage/masiao/sage-5.0.beta8-patchbot/devel/sage-11593/sage/rings/function_field/function_field_element.pyx", line 445:
sage: a.list()
Expected:
[(2*x^3 + x)/(16*x^5 + 2*x^2 + 1), -x^2/(8*x^5 + x^2 + 1/2)]
Got:
[(1/2*x^3 + 1/4*x)/(4*x^5 + 1/2*x^2 + 1/4), -1/2*x^2/(4*x^5 + 1/2*x^2 + 1/4)]
**********************************************************************
```

### comment:13 Changed 10 years ago by Kwankyu Lee

Dependencies: → #12612 needs_work → needs_review

The new code also speeds up things. See below

Before the patch,

```sage: sage: S.<x>=QQ[]
sage: sage: fS=FractionField(S)
sage: sage: R.<y>=fS[]
sage: sage: f=y^20
sage: sage: g=y^5+1/x*y^4
sage: sage: timeit("f.quo_rem(g)")
125 loops, best of 3: 3.25 ms per loop
```

After the patch,

```sage: sage: S.<x>=QQ[]
sage: sage: fS=FractionField(S)
sage: sage: R.<y>=fS[]
sage: sage: f=y^20
sage: sage: g=y^5+1/x*y^4
sage: sage: timeit("f.quo_rem(g)")
125 loops, best of 3: 1.75 ms per loop
```

### comment:14 Changed 10 years ago by Kwankyu Lee

Work issues: fails doctests

Fixed the doctest failure issues.

### comment:15 follow-up:  16 Changed 10 years ago by Frédéric Chapoton

Maybe one could consider adding documentation for the procedure cdef _inplace_truncate ?

You are changing something inside this procedure, so the correct behaviour should be tested.

### comment:16 in reply to:  15 Changed 10 years ago by Kwankyu Lee

Maybe one could consider adding documentation for the procedure cdef _inplace_truncate ?

You are changing something inside this procedure, so the correct behaviour should be tested.

I corrected a bug in _inplace_truncate, which is of only internal use, unlike the truncate method defined just above it. It's correct behaviour is indirectly checked by other methods that use it in various places. On the other hand, the procedure is quite self-explanatory by its name and the short code, and I doubt the value of a documentation for it.

Last edited 10 years ago by Kwankyu Lee (previous) (diff)

### comment:17 follow-up:  18 Changed 10 years ago by Frédéric Chapoton

Well, I understand. But it would have been one little step towards the official goal of 100% doctesting. The question is not so much the value of the documentation, that its mere existence, even if it is not very useful..

### comment:18 in reply to:  17 Changed 10 years ago by Kwankyu Lee

Well, I understand. But it would have been one little step towards the official goal of 100% doctesting. The question is not so much the value of the documentation, that its mere existence, even if it is not very useful..

I found out that as `_inplace_truncate` is only a `cdef` method, that is not even counted for the doctest coverage. See the result of

```sage -coverage sage/rings/polynomial/polynomial_element.pyx
```

### comment:19 Changed 10 years ago by Frédéric Chapoton

Status: needs_review → positive_review

ok for me, positive review

### comment:20 Changed 10 years ago by Jeroen Demeyer

Milestone: sage-5.4 → sage-5.5 → Frédéric Chapoton

### comment:21 Changed 10 years ago by Jeroen Demeyer

Merged in: → sage-5.5.beta1 → fixed positive_review → closed

### comment:22 Changed 10 years ago by Jeroen Demeyer

sage-5.5.beta0 + #11593 gives

```sage -t --long "devel/sage/sage/schemes/elliptic_curves/ell_number_field.py"
The doctested process was killed by signal 11
[166.4 s]
```

on OS X 10.4 PPC (not on other systems as far as I know).

I think the real issue is with #715+#11521, not this ticket.

### comment:23 follow-up:  27 Changed 10 years ago by Jeroen Demeyer

Besides, why `TypeError` when dividing by a non-unit? It seems to me that `TypeError` is not the right exception for this, I believe `ArithmeticError` suits better.

### comment:24 Changed 10 years ago by Jeroen Demeyer

Resolution: fixed closed → new

And exceptions should be doctested too.

### comment:25 Changed 10 years ago by Jeroen Demeyer

Merged in: sage-5.5.beta1 new → needs_review

### comment:26 Changed 10 years ago by Jeroen Demeyer

Keywords: fix and doctest exceptions added needs_review → needs_work

### comment:27 in reply to:  23 Changed 10 years ago by Kwankyu Lee

Besides, why `TypeError` when dividing by a non-unit? It seems to me that `TypeError` is not the right exception for this, I believe `ArithmeticError` suits better.

`TypeError` was my inevitable choice to avoid triggering doctest failures in other parts of Sage, which depend on `TypeError` exception for normal operation. I need to remind myself of the details how and where that happens.

### comment:28 Changed 10 years ago by Kwankyu Lee

The new patch raises `ArithmeticError` for nonunit divisor

### comment:29 Changed 10 years ago by Kwankyu Lee

Status: needs_work → needs_review

### comment:30 follow-up:  31 Changed 10 years ago by Frédéric Chapoton

maybe the second and third tests at the beginning should rather be swapped ? What do we expect when we divide 0 by something which does not have an invertible leading coefficient ?

### comment:31 in reply to:  30 Changed 10 years ago by Kwankyu Lee

maybe the second and third tests at the beginning should rather be swapped ? What do we expect when we divide 0 by something which does not have an invertible leading coefficient ?

Good point! The tests are swapped in a new patch.

### comment:32 Changed 10 years ago by Frédéric Chapoton

Status: needs_review → positive_review

well, let me give again a positive review. Maybe the long-test problem is still there, but it does not appear on 5.4rc3.

### comment:33 Changed 10 years ago by Jeroen Demeyer

Milestone: sage-5.5 → sage-5.6

### comment:34 Changed 10 years ago by Jeroen Demeyer

Status: positive_review → needs_work

With #8992, I get

```**********************************************************************
File "/release/merger/sage-5.6.beta0/devel/sage-main/sage/rings/polynomial/polynomial_quotient_ring.py", line 407:
sage: Q3(Q1.gen())
Exception raised:
Traceback (most recent call last):
File "/release/merger/sage-5.6.beta0/local/bin/ncadoctest.py", line 1231, in run_one_test
self.run_one_example(test, example, filename, compileflags)
File "/release/merger/sage-5.6.beta0/local/bin/sagedoctest.py", line 38, in run_one_example
OrigDocTestRunner.run_one_example(self, test, example, filename, compileflags)
File "/release/merger/sage-5.6.beta0/local/bin/ncadoctest.py", line 1172, in run_one_example
compileflags, 1) in test.globs
File "<doctest __main__.example_5[28]>", line 1, in <module>
Q3(Q1.gen())###line 407:
sage: Q3(Q1.gen())
File "parent.pyx", line 800, in sage.structure.parent.Parent.__call__ (sage/structure/parent.c:7268)
File "parent.pyx", line 2153, in sage.structure.parent.Parent.convert_map_from (sage/structure/parent.c:15082)
File "parent.pyx", line 2165, in sage.structure.parent.Parent.discover_convert_map_from (sage/structure/parent.c:15301)
File "parent.pyx", line 1903, in sage.structure.parent.Parent.coerce_map_from (sage/structure/parent.c:14051)
File "parent.pyx", line 1974, in sage.structure.parent.Parent.coerce_map_from (sage/structure/parent.c:13804)
File "parent.pyx", line 2068, in sage.structure.parent.Parent.discover_coerce_map_from (sage/structure/parent.c:14231)
File "parent_old.pyx", line 507, in sage.structure.parent_old.Parent._coerce_map_from_ (sage/structure/parent_old.c:6428)
File "/release/merger/sage-5.6.beta0/local/lib/python/site-packages/sage/rings/polynomial/polynomial_quotient_ring.py", line 485, in _coerce_map_from_
if not self.__polynomial.divides(R.modulus()):
File "element.pyx", line 2027, in sage.structure.element.CommutativeRingElement.divides (sage/structure/element.c:16725)
File "element.pyx", line 2021, in sage.structure.element.CommutativeRingElement.divides (sage/structure/element.c:16636)
File "polynomial_element.pyx", line 1724, in sage.rings.polynomial.polynomial_element.Polynomial.__mod__ (sage/rings/polynomial/polynomial_element.c:16751)
_, R = self.quo_rem(other)
File "element.pyx", line 3223, in sage.structure.element.NamedBinopMethod.__call__ (sage/structure/element.c:24763)
File "polynomial_element.pyx", line 6478, in sage.rings.polynomial.polynomial_element.Polynomial_generic_dense.quo_rem (sage/rings/polynomial/polynomial_element.c:47106)
**********************************************************************
```
Last edited 10 years ago by Jeroen Demeyer (previous) (diff)

### comment:35 Changed 10 years ago by Frédéric Chapoton

Well, this failing test does not make sense to me. Why on earth should we be able to convert

```x as element of the ring Q[x]/(x^6 - x^4 - 5*x^2 - 3)

to

x as element of the ring Q[x][y]/(y^2+1)
```

I propose to remove this test. Anybody else has an opinion ?

### comment:36 Changed 10 years ago by Jeroen Demeyer

I think the doctest explains it pretty well: it is a conversion from `Q[x]/(x^6 - x^4 - 5*x^2 - 3)` to `Q[x]` which matters.

You can ask questions at #8992, where this test was introduced.

### Changed 10 years ago by Kwankyu Lee

rebased for Sage 5.6

### comment:37 Changed 10 years ago by Kwankyu Lee

Actually the fault was with #8992. There is now no need to take off the problematic doctest of #8992.

### comment:38 Changed 10 years ago by Kwankyu Lee

The faulty part of #8992 is now remedied and included in the current patch for this ticket.

### comment:39 Changed 10 years ago by Kwankyu Lee

Status: needs_work → needs_review

### comment:40 Changed 10 years ago by Frédéric Chapoton

Status: needs_review → positive_review

ok for me, positive review

### comment:41 Changed 10 years ago by Jeroen Demeyer

Merged in: → sage-5.8.beta2 → fixed positive_review → closed
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