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IntegerModRing representation

asked 2010-10-10 03:35:27 -0500

czsan gravatar image

How can I generate the elements of IntegerModring(n) in symmetric representation? For example Integers(6) = {0, 1,-1,2, -2, 3}

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answered 2010-11-09 06:43:16 -0500

lftabera gravatar image

Well, if you want to change the representation of the objects, so when you type Integers(6)(5) it returns -1, I think that it is not implemented in sage.

However, if computing the symmetric representant of an element in Integers(n) can be done with the following code:

I use this function in some multimodular algorithms.

def lift_pm(p):  
"""
Compute a representative of the element p mod n in [-n/2 , n/2]

INPUT: 

- ``p`` an integer mod n

OUTPUT:

- An integer r such that  -n/2 < r <= n/2

EXAMPLES::

    sage: p = Mod(2,4)
    sage: lift_pm(p)
    2
    sage: p = Mod(3,4)
    sage: lift_pm(p)
    -1
"""
r = p.lift()
if r*2 > p.modulus():
    r -= p.modulus()
return r
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Thank you, this is close to my wish. I will use it.

czsan gravatar imageczsan ( 2010-11-13 21:16:07 -0500 )edit

The true solution would be like in Maple : `mod`:=mods

czsan gravatar imageczsan ( 2010-11-13 22:20:19 -0500 )edit
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answered 2013-10-06 21:28:41 -0500

czsan gravatar image

Surprisingly in IntegerModrings Sage uses positive representations, but in polynomialrings over these it uses symmetric representation. So we can make the computations with the residues in the polynomialring.

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This is true, if the polynomialring is multivariate

czsan gravatar imageczsan ( 2013-10-07 01:26:48 -0500 )edit

and only if the modulus is prime?

czsan gravatar imageczsan ( 2013-10-07 08:50:57 -0500 )edit
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answered 2010-10-10 08:36:17 -0500

niles gravatar image

You could use Python's list comprehension to make a list of the numbers you want:

sage: N = 5
sage: [(-1)^(n+1)*floor((n+1)/2) for n in range(N)]
[0, 1, -1, 2, -2]
sage: N = 6
sage: [(-1)^(n+1)*floor((n+1)/2) for n in range(N)]
[0, 1, -1, 2, -2, 3]

Is this the kind of thing you were looking for, or something else?

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Thank you. It's a good idea. I believed that the IntegerModRing have a special option.

czsan gravatar imageczsan ( 2010-10-10 11:26:59 -0500 )edit

But it is not teh solition. I think, My question was'nt clear. The positive representation for mod 6 is {1,2,3,4,5,6}. The symmetric representation is {1,2,3,-2,-1}. The problem is'nt that, how can generate this remainder set, but the Integers(6) ring how can use it permanently.

czsan gravatar imageczsan ( 2010-10-10 13:04:52 -0500 )edit

oh, sorry, I don't know how to do that.

niles gravatar imageniles ( 2010-10-11 02:05:33 -0500 )edit

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Asked: 2010-10-10 03:35:27 -0500

Seen: 871 times

Last updated: Oct 06 '13