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Order of coefficients of univariate polynomial: Bug or Feature?

asked 2022-05-05 10:14:31 +0200

Matthias Steiner gravatar image

I obsevered the following behaviour in SageMath 9.3 and 9.5.

sage: P.<x> = PolynomialRing(QQ)
sage: f = 1 + 2 * x + 3 * x**2
sage: f.coefficients()
[1, 2, 3]
sage: f.monomials()
[x^2, x, 1]

So the coefficients of $f$ are sorted in ascending order while the monomials are sorted in descending order. Is this a bug or a feature, because for multivariate polynomials coefficients and monomials are sorted in descending order with respect to the chosen monomial order?

sage: P.<x, y> = PolynomialRing(QQ)
sage: f = 1 + 2 * x + 3 * y + 4 * x * y
sage: f.coefficients()
[4, 2, 3, 1]
sage: f.monomials()
[x*y, x, y, 1]
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That inconsistent behavior in the univariate case looks undesirable to me, but changing it would break compatibility with existing code (if anyone ever used it). As a workaround you can use P.<x> = PolynomialRing(QQ, 1) to create a multivariate polynomial ring with one variable.

rburing gravatar imagerburing ( 2022-05-05 12:01:23 +0200 )edit
3

If the inconsistency is not changed, then it should at least be mentioned in the documentation. It took me quite a while to figure out why my code did not produced the desired result.

Matthias Steiner gravatar imageMatthias Steiner ( 2022-05-05 12:59:16 +0200 )edit
2

It also may be unexpected that if f = 1 + 2*x**2, then f.coefficients() will return (1, 2): by default it only includes nonzero coefficients. Note that f.exponents() returns information consistent with f.coefficients(). Maybe they're supposed to be used together?

John Palmieri gravatar imageJohn Palmieri ( 2022-05-05 18:21:19 +0200 )edit
2

I created https://trac.sagemath.org/ticket/33813 to improve the documentation for these methods.

John Palmieri gravatar imageJohn Palmieri ( 2022-05-06 01:11:14 +0200 )edit

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answered 2022-05-06 19:13:44 +0200

dan_fulea gravatar image

I will say some words here, not in a comment, since the space is more generous. First of all, methods implemented for some objects are often implemented with a special purpose. They may be used in further implemented algorithms, or are implemented to provide a "library usage". The questions assumes tacitly, that the two methods coefficients and monomials of the object f, that i chose in a different way

sage: P.<x> = PolynomialRing(QQ)
sage: f = 11*x^3 - 17*x^7 + 56*x^10 + 9*x^5
sage: f.parent()
Univariate Polynomial Ring in x over Rational Field
sage: type(f)
<class 'sage.rings.polynomial.polynomial_rational_flint.Polynomial_rational_flint'>

should be related to each other. No, in this case the programmer expects one method. Let us ask for the coefficients and the monomials involved:

sage: f.coefficients()
[11, 9, -17, 56]
sage: f.monomials()
[x^10, x^7, x^5, x^3]

In the doc string of the last method we have the information:

sage: f.monomials??

::::::::::::: many lines then... :::::::::::::

Source:   
    def monomials(self):
        """
        Return the list of the monomials in ``self`` in a decreasing order of their degrees.

        EXAMPLES::

            sage: P.<x> = QQ[]
            sage: f = x^2 + (2/3)*x + 1

so it is an intention to sort in this manner. For the coefficients, the natural, "expected" behavior is kept, we can for instance ask for:

sage: f.coefficients()
[11, 9, -17, 56]
sage: f.coefficients(sparse=True)
[11, 9, -17, 56]
sage: f.coefficients(sparse=False)
[0, 0, 0, 11, 0, 9, 0, -17, 0, 0, 56]
sage: f.list()
[0, 0, 0, 11, 0, 9, 0, -17, 0, 0, 56]
sage: f.dict()
{3: 11, 5: 9, 7: -17, 10: 56}

to extract in a way or an other the information on the polynomial.

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Asked: 2022-05-05 10:14:31 +0200

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Last updated: May 06