Opened 7 years ago
Last modified 4 years ago
#12179 needs_work defect
Binomial of integer (mod n) returns integer
Reported by: | scotts | Owned by: | AlexGhitza |
---|---|---|---|
Priority: | major | Milestone: | sage-6.4 |
Component: | basic arithmetic | Keywords: | binomial coefficient modulo sd35 |
Cc: | Merged in: | ||
Authors: | Sam Scott, Marco Streng | Reviewers: | Colton Pauderis, Johan Bosman, Marco Streng |
Report Upstream: | N/A | Work issues: | |
Branch: | Commit: | ||
Dependencies: | Stopgaps: | todo |
Description (last modified by )
sage: R = Integers(6) sage: binomial(R(5), R(2)) 10 sage: binomial(R(5), R(2)).parent() Integer Ring
But binomial(R(5), R(2))
is nonsense, both as an element of ZZ and as an element of R:
sage: binomial(5, 2) 10 sage: binomial(11, 2) 55 sage: binomial(5, 8) 0
On input binomial(x, y)
, what Sage should do instead is the following:
- If the parent of y is Zmod(n) rather than ZZ, a
TypeError
should be raised. - (This seems to be fixed by #17852) If factorial(y) is zero or a zero-divisor in the parent of x, a
ZeroDivisionError
should be raised. This is automatic if one computes binomial(x, y) simply asx.parent()(prod([x-k for k in range(y)]) / factorial(y))
Apply:
Attachments (4)
Change History (31)
Changed 7 years ago by
comment:1 Changed 7 years ago by
- Status changed from new to needs_review
comment:2 Changed 7 years ago by
- Status changed from needs_review to positive_review
Looks good to me: positive review.
comment:3 Changed 7 years ago by
Changed 7 years ago by
comment:4 Changed 7 years ago by
Patch added to deal with the case where the input was a primitive python int.
Thanks to Colton (cpauderis) for catching that one.
comment:5 Changed 7 years ago by
- Status changed from positive_review to needs_work
The line
Test of integers modulo n: }}{ should end in two colons (for docbuilding). Furthermore, {{{ return x.parent()(binomial(ZZ(x), m, **kwds)) }}} is inefficient when computing modulo a small number n: the binomial coefficient over ZZ may be huge, whereas its reduction modulo n will be small. It is therefore better to to the entire calculation modulo n.
comment:6 follow-up: ↓ 9 Changed 7 years ago by
The line
Test of integers modulo n:
should end in two colons (for docbuilding). Furthermore,
return x.parent()(binomial(ZZ(x), m, **kwds))
is inefficient when computing modulo a small number n: the binomial coefficient over ZZ may be huge, whereas its reduction modulo n will be small. It is therefore better to to the entire calculation modulo n.
comment:7 Changed 7 years ago by
- Keywords sd35 added
comment:8 Changed 7 years ago by
- Dependencies set to #11417
- Keywords coefficient added; coefficiant removed
- Reviewers set to Colton Pauderis, Johan Bosman, Marco Streng
- Work issues set to rebase on top of #11417, reST formatting issue, improve efficiency
Patch is incompatible with #11417, which is already merged.
comment:9 in reply to: ↑ 6 ; follow-up: ↓ 11 Changed 7 years ago by
Replying to johanbosman:
Furthermore,
return x.parent()(binomial(ZZ(x), m, **kwds))is inefficient when computing modulo a small number n: the binomial coefficient over ZZ may be huge, whereas its reduction modulo n will be small. It is therefore better to to the entire calculation modulo n.
I definitely agree, the changes I was proposing were to simply try and return a more sensible type.
Since I am new to sage, I was trying to keep as much of the existing algorithm intact, so that it wouldn't have any unexpected behaviour. For example, currently it seems that sage uses Peri to calculate the usual binomial coefficient. So I wasn't sure if the implementation for modulo n would belong here.
comment:10 Changed 7 years ago by
That should obviously be Pari, not Peri.
comment:11 in reply to: ↑ 9 Changed 7 years ago by
Replying to scotts:
I definitely agree, the changes I was proposing were to simply try and return a more sensible type.
Since I am new to sage, I was trying to keep as much of the existing algorithm intact, so that it wouldn't have any unexpected behaviour. For example, currently it seems that sage uses Peri to calculate the usual binomial coefficient. So I wasn't sure if the implementation for modulo n would belong here.
That sounds sensible, this is a bugfix patch and the inefficiency is not introduced by your patch. So if you don't want to, you don't have to improve the speed. However, since you're editing anyway, you might as well. It's up to you. The other issues
- documentation that does not build correctly
- a conflicting patch has already been merged
do need to be resolved though.
For the first issue, after building Sage, use sage -docbuild reference html
to rebuild the documentation and see if all the changed documentation looks good (in this case, replacing : by :: will probably be enough).
As for the second issue, you should not write two independent patches editing the same part of a file. They can't be applied after each other, because the second-to-be-applied can't find the piece of code that it should change. You should write your patch for #12179 on top of a copy of Sage that has #11417 applied.
Changed 6 years ago by
comment:12 Changed 6 years ago by
- Status changed from needs_work to needs_review
Finally got around to reinstalling sage after a hard-drive failure. Hoping to get back on track with contributing to sage. Sorry for the incredibly slow response.
Hopefully this should tie up this loose end.
While I do agree that there is room for improving the speed of calculating binomial coefficients mod N, I don't feel like it's worth bloating the binomial function with multiple extra lines of code for something which doesn't seem to be used much.
Perhaps it could be added as a feature at a later stage when the need is there.
However, I feel that this patch adequately addresses the original issue: that elements are treated as integers and returned as such, with no attempt to return them to their original class. This could potentially help with other cases.
Thanks for the advice with regards to the other issues.
comment:13 follow-ups: ↓ 14 ↓ 22 Changed 6 years ago by
- Description modified (diff)
- Status changed from needs_review to needs_work
- Work issues rebase on top of #11417, reST formatting issue, improve efficiency deleted
Oops, I never got around to looking at the mathematics while reviewing your rest-formatting before. The problem is that binomial(x, y) is in general not defined for x and y integers modulo n. I'll update the ticket description appropriately.
Allowing y to be anything other than an integer makes no sense to me. What would the definition be? In any case, the output has to be in the ring containing x, and will definitely depend on the exact integer y, and not just on y modulo any n. For example:
sage: binomial(5,-2) # bionomial(5, Zmod(2)(0)) = ??? 0 sage: binomial(5,0) 1 sage: binomial(5,2) 10 sage: binomial(5,4) 5 sage: binomial(5,6) 0
Summary: if y is an element of Zmod(n), then surely a ValueError
must be raised.
The situation when x is an element of Zmod(n) is more complicated, but cannot be always automatically allowed:
sage: binomial(1, 3) % 3 # binomial(Zmod(3)(1), 3) = ??? 0 sage: binomial(4, 3) % 3 1 sage: binomial(7, 3) % 3 2
So the correct answer to binomial(Zmod(3)(1), 3)
is "any integer modulo 3". Just returning Zmod(3)(0) is wrong. It would be ok to raise an error, I'd say ZeroDivisionError
, because of the following completely general formula:
sage: binomial(x, 3) 1/6*x^3 - 1/2*x^2 + 1/3*x
There are of course some cases that we can still allow. Suppose the input is binomial(Zmod(n)(x), y)
.
- If n is coprime to factorial(y), then all divisions are allowed, so the output should again be a well-defined element of Zmod(n). This is what happens with the code
x.parent()(prod([x-k for k in range(y)]) / factorial(y))
from the ticket description. - In general, we could allow the output be an element of Zmod(n / gcd(factorial(y), n)) instead of Zmod(n). This would mean no
ZeroDivisionError
s are raised, but Zmod(1)(0) is returned instead. This makes sense in the theory and practice of p-adic power series and formal groups, but may not be what the user expects, so I left this out of the new ticket description.
comment:14 in reply to: ↑ 13 Changed 6 years ago by
- Description modified (diff)
Replying to mstreng:
Summary: if y is an element of Zmod(n), then surely a
ValueError
must be raised.
no, wait, I meant TypeError
comment:15 Changed 6 years ago by
- Description modified (diff)
comment:16 Changed 6 years ago by
- Dependencies #11417 deleted
- Description modified (diff)
apply only 12179_new.patch
Changed 6 years ago by
comment:17 Changed 6 years ago by
New version, but it screws up lots of symbolic things. Perhaps a few special cases, like binomial(n, n) always returning 1, will fix this. I'm done with this for now.
comment:18 Changed 6 years ago by
- Milestone changed from sage-5.11 to sage-5.12
comment:19 Changed 5 years ago by
- Milestone changed from sage-6.1 to sage-6.2
comment:20 Changed 5 years ago by
- Milestone changed from sage-6.2 to sage-6.3
comment:21 Changed 5 years ago by
- Milestone changed from sage-6.3 to sage-6.4
comment:22 in reply to: ↑ 13 Changed 5 years ago by
Replying to mstreng:
So the correct answer to
binomial(Zmod(3)(1), 3)
is "any integer modulo 3". Just returning Zmod(3)(0) is wrong. It would be ok to raise an error, I'd sayZeroDivisionError
Pari does this too.
There are of course some cases that we can still allow. Suppose the input is
binomial(Zmod(n)(x), y)
.
Example with Pari:
? binomial(Mod(7,11),3) %3 = Mod(2, 11)
comment:23 follow-up: ↓ 24 Changed 4 years ago by
- Milestone changed from sage-6.4 to sage-duplicate/invalid/wontfix
- Status changed from needs_work to needs_review
Hello,
I just discover this ticket. During a cleanup in sage.rings.arith
(#17852) I took care of this case. I propose to close this one as duplicate. With the branch applied we got
sage: from sage.rings.arith import binomial sage: R = Integers(6) sage: binomial(R(5), R(2)) Traceback (most recent call last): ... ZeroDivisionError: Inverse does not exist. sage: R = Integers(21) sage: binomial(R(5), R(2)) 10 sage: binomial(R(5), R(2)).parent() Ring of integers modulo 21
Vincent
comment:24 in reply to: ↑ 23 ; follow-up: ↓ 25 Changed 4 years ago by
- Description modified (diff)
- Milestone changed from sage-duplicate/invalid/wontfix to sage-6.4
- Status changed from needs_review to needs_work
Replying to vdelecroix:
sage: R = Integers(21) sage: binomial(R(5), R(2)) 10
This should be TypeError
, because binomial(x,y)
makes no sense when y is an element of R
. It only makes sense when y is an integer. For example, binomial(5, 2) = 10, but binomial(5, 2+21) = 0.
So of the two points in the ticket description, the work in #17852 fixes the second one, but the first one is still open.
comment:25 in reply to: ↑ 24 Changed 4 years ago by
Replying to mstreng:
Replying to vdelecroix:
sage: R = Integers(21) sage: binomial(R(5), R(2)) 10
This should be
TypeError
, becausebinomial(x,y)
makes no sense when y is an element ofR
. It only makes sense when y is an integer. For example, binomial(5, 2) = 10, but binomial(5, 2+21) = 0.So of the two points in the ticket description, the work in #17852 fixes the second one, but the first one is still open.
Right!
comment:26 Changed 4 years ago by
Hi,
Ticket #17852 is in pass to be positively reviewed. Let me summarize what will change when calling rings.arith.binomial(x,y)
:
y
must be an integer (actually, I only asked thatZZ(y)
does work and the first lines of code doy = ZZ(y)
)- the output type is always the type of
x
- if
factorial(y)
is not invertible aZeroDivisionError
is raised (I checked the behavior on many finite rings that I was able to think of)
Vincent
comment:27 Changed 4 years ago by
- Stopgaps set to todo
Attached patch should add this functionality with no impact on speed of standard integer/float/etc binomial calculation.