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Problem with the minus sign in symbolic expression

asked 2016-12-08 10:15:46 +0200

updated 2016-12-08 11:24:07 +0200

Hi all!

My sage version is:

'SageMath Version 6.9, Release Date: 2015-10-10'

In my code, I have a multivariate polynomial with a sign symbol which doesn't return the correct result. I.e.

The code snippet is as follows:

A1 = [(', '.join('x%i'%i for i in [1.. n]))]; ### construct a suitable multivariate ring
 V = var(A1[0])                               ### define a str variable
x=vector(list(V))                                 ### convert to vector

a1 and b1 are returned from another function.
types of a1 and x are different:

 type(a1)=<type 'sage.modules.vector_modn_dense.Vector_modn_dense'>

type(x)=  <class 'sage.modules.vector_symbolic_dense.FreeModule_ambient_field_with_category.element_class'>

Could this be a problem?

a1 is a vector with values (76, 83), b1 is scalar with value 62, x is a vector with values (x1,x2). Thus, f1 should return 62-76 * x1-83 * x2; however it returns 25x1 + 18x2 + 62 which is not correct.

Please note that if I use addition in f1 as follows:

sage: f1=b1+a1.dot_product(x)

The result is correct,i.e. 76x1 + 83x2 + 62

type(f1) is sage.symbolic.expression.Expression

What could the problem be? I tried to convert the symbolic expression to multivariate polynomial, but this didn't work either? I also though there might be a problem with the minus symbol in my PC but couldn't solve it this way either.

Thank you for your responses! Regards, Natassa

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Please edit your question, adding the actual code you use to define a1, b1, and x.

Also, please say which version of Sage you are using.

slelievre gravatar imageslelievre ( 2016-12-08 11:04:24 +0200 )edit

For reference, this was also asked on sage-support.

slelievre gravatar imageslelievre ( 2016-12-08 14:23:31 +0200 )edit

1 Answer

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answered 2016-12-08 11:12:48 +0200

slelievre gravatar image

updated 2016-12-08 11:55:56 +0200

I suppose you are working modulo 101.

Compare the following.

For reference I am working with:

sage: version()
'SageMath version 7.4, Release Date: 2016-10-18'

Using plain integers.

sage: a1 = vector((76, 83))
sage: b1 = 62
sage: x1, x2 = SR.var('x1 x2')
sage: x = vector((x1, x2))
sage: b1 - a1.dot_product(x)
-76*x1 - 83*x2 + 62
sage: b1 + a1.dot_product(x)
76*x1 + 83*x2 + 62

Using integers modulo 101.

sage: R = Zmod(101)
sage: a1 = vector(R, (76, 83))
sage: b1 = R(62)
sage: x1, x2 = SR.var('x1 x2')
sage: x = vector((x1, x2))
sage: b1 - a1.dot_product(x)
25*x1 + 18*x2 + 62
sage: b1 + a1.dot_product(x)
76*x1 + 83*x2 + 62

If you get a1 and b1 as above, or from another function and they are defined modulo 101, you can change ring as follows.

sage: a1 = a1.change_ring(ZZ)
sage: b1 = ZZ(b1)

Then this works as expected:

sage: b1 - a1.dot_product(x)
-76*x1 - 83*x2 + 62
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Indeed, now I see what the problem was. Thank you!

Natassa Theodouli gravatar imageNatassa Theodouli ( 2016-12-08 11:40:08 +0200 )edit

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Asked: 2016-12-08 10:15:46 +0200

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Last updated: Dec 08 '16