Optimized Quadratic Number Field Elements¶

This file defines a Cython class NumberFieldElement_quadratic to speed up computations in quadratic extensions of \(\QQ\).

AUTHORS:

Robert Bradshaw (2007-09): Initial version
David Harvey (2007-10): fix up a few bugs, polish around the edges
David Loeffler (2009-05): add more documentation and tests
Vincent Delecroix (2012-07): comparisons for quadratic number fields (trac ticket #13213), abs, floor and ceil functions (trac ticket #13256)

Todo

The _new() method should be overridden in this class to copy the D and standard_embedding attributes

class sage.rings.number_field.number_field_element_quadratic.NumberFieldElement_quadratic¶

Bases: sage.rings.number_field.number_field_element.NumberFieldElement_absolute

A NumberFieldElement_quadratic object gives an efficient representation of an element of a quadratic extension of \(\QQ\).

Elements are represented internally as triples \((a, b, c)\) of integers, where \({\rm gcd}(a, b, c) = 1\) and \(c > 0\), representing the element \((a + b \sqrt{D}) / c\). Note that if the discriminant \(D\) is \(1 \bmod 4\), integral elements do not necessarily have \(c = 1\).

ceil()¶

Returns the ceil.

EXAMPLES:

sage: K.<sqrt7> = QuadraticField(7, name='sqrt7')
sage: sqrt7.ceil()
3
sage: (-sqrt7).ceil()
-2
sage: (1022/313*sqrt7 - 14/23).ceil()
9

charpoly(var='x')¶

The characteristic polynomial of this element over \(\QQ\).

EXAMPLES:

sage: K.<a> = NumberField(x^2-x+13)
sage: a.charpoly()
x^2 - x + 13
sage: b = 3-a/2
sage: f = b.charpoly(); f
x^2 - 11/2*x + 43/4
sage: f(b)
0

continued_fraction()¶

Return the (finite or ultimately periodic) continued fraction of self.

EXAMPLES:

sage: K.<sqrt2> = QuadraticField(2)
sage: cf = sqrt2.continued_fraction(); cf
[1; (2)*]
sage: cf.n()
1.41421356237310
sage: sqrt2.n()
1.41421356237309
sage: cf.value()
sqrt2

sage: (sqrt2/3 + 1/4).continued_fraction()
[0; 1, (2, 1, 1, 2, 3, 2, 1, 1, 2, 5, 1, 1, 14, 1, 1, 5)*]

continued_fraction_list()¶

Return the preperiod and the period of the continued fraction expansion of self.

EXAMPLES:

sage: K.<sqrt2> = QuadraticField(2)
sage: sqrt2.continued_fraction_list()
((1,), (2,))
sage: (1/2+sqrt2/3).continued_fraction_list()
((0, 1, 33), (1, 32))

For rational entries a pair of tuples is also returned but the second one is empty:

sage: K(123/567).continued_fraction_list()
((0, 4, 1, 1, 1, 1, 3, 2), ())

denominator()¶

Return the denominator of self. This is the LCM of the denominators of the coefficients of self, and thus it may well be \(> 1\) even when the element is an algebraic integer.

EXAMPLES:

sage: K.<a> = NumberField(x^2+x+41)
sage: a.denominator()
1
sage: b = (2*a+1)/6
sage: b.denominator()
6
sage: K(1).denominator()
1
sage: K(1/2).denominator()
2
sage: K(0).denominator()
1

sage: K.<a> = NumberField(x^2 - 5)
sage: b = (a + 1)/2
sage: b.denominator()
2
sage: b.is_integral()
True

floor()¶

Returns the floor of x.

EXAMPLES:

sage: K.<sqrt2> = QuadraticField(2,name='sqrt2')
sage: sqrt2.floor()
1
sage: (-sqrt2).floor()
-2
sage: (13/197 + 3702/123*sqrt2).floor()
42
sage: (13/197-3702/123*sqrt2).floor()
-43

galois_conjugate()¶

Return the image of this element under action of the nontrivial element of the Galois group of this field.

EXAMPLES:

sage: K.<a> = QuadraticField(23)
sage: a.galois_conjugate()
-a

sage: K.<a> = NumberField(x^2 - 5*x + 1)
sage: a.galois_conjugate()
-a + 5
sage: b = 5*a + 1/3
sage: b.galois_conjugate()
-5*a + 76/3
sage: b.norm() ==  b * b.galois_conjugate()
True
sage: b.trace() ==  b + b.galois_conjugate()
True

imag()¶

Returns the imaginary part of self.

EXAMPLES:

sage: K.<sqrt2> = QuadraticField(2)
sage: sqrt2.imag()
0
sage: parent(sqrt2.imag())
Rational Field

sage: K.<i> = QuadraticField(-1)
sage: i.imag()
1
sage: parent(i.imag())
Rational Field

sage: K.<a> = NumberField(x^2 + x + 1, embedding=CDF.0)
sage: a.imag()
1/2*sqrt3
sage: a.real()
-1/2
sage: SR(a)
1/2*I*sqrt(3) - 1/2
sage: bool(I*a.imag() + a.real() == a)
True

is_integer()¶

Check whether this number field element is an integer.

See also

is_integer() to test if this element is an integer
is_integral() to test if this element is an algebraic integer

EXAMPLES:

sage: K.<sqrt3> = QuadraticField(3)
sage: sqrt3.is_rational()
False
sage: (sqrt3-1/2).is_rational()
False
sage: K(0).is_rational()
True
sage: K(-12).is_rational()
True
sage: K(1/3).is_rational()
True

minpoly(var='x')¶

The minimal polynomial of this element over \(\QQ\).

INPUT:

var – the minimal polynomial is defined over a polynomial ring in a variable with this name. If not specified this defaults to x.

EXAMPLES:

sage: K.<a> = NumberField(x^2+13)
sage: a.minpoly()
x^2 + 13
sage: a.minpoly('T')
T^2 + 13
sage: (a+1/2-a).minpoly()
x - 1/2

norm(K=None)¶

Return the norm of self. If the second argument is None, this is the norm down to \(\QQ\). Otherwise, return the norm down to K (which had better be either \(\QQ\) or this number field).

EXAMPLES:

sage: K.<a> = NumberField(x^2-x+3)
sage: a.norm()
3
sage: a.matrix()
[ 0  1]
[-3  1]
sage: K.<a> = NumberField(x^2+5)
sage: (1+a).norm()
6

The norm is multiplicative:

sage: K.<a> = NumberField(x^2-3)
sage: a.norm()
-3
sage: K(3).norm()
9
sage: (3*a).norm()
-27

We test that the optional argument is handled sensibly:

sage: (3*a).norm(QQ)
-27
sage: (3*a).norm(K)
3*a
sage: (3*a).norm(CyclotomicField(3))
Traceback (most recent call last):
...
ValueError: no way to embed L into parent's base ring K

numerator()¶

Return self*self.denominator().

EXAMPLES:

sage: K.<a> = NumberField(x^2+x+41)
sage: b = (2*a+1)/6
sage: b.denominator()
6
sage: b.numerator()
2*a + 1

parts()¶

This function returns a pair of rationals \(a\) and \(b\) such that self \(= a+b\sqrt{D}\).

This is much closer to the internal storage format of the elements than the polynomial representation coefficients (the output of self.list()), unless the generator with which this number field was constructed was equal to \(\sqrt{D}\). See the last example below.

EXAMPLES:

sage: K.<a> = NumberField(x^2-13)
sage: K.discriminant()
13
sage: a.parts()
(0, 1)
sage: (a/2-4).parts()
(-4, 1/2)
sage: K.<a> = NumberField(x^2-7)
sage: K.discriminant()
28
sage: a.parts()
(0, 1)
sage: K.<a> = NumberField(x^2-x+7)
sage: a.parts()
(1/2, 3/2)
sage: a._coefficients()
[0, 1]

real()¶

Returns the real part of self, which is either self (if self lives it a totally real field) or a rational number.

EXAMPLES:

sage: K.<sqrt2> = QuadraticField(2)
sage: sqrt2.real()
sqrt2
sage: K.<a> = QuadraticField(-3)
sage: a.real()
0
sage: (a + 1/2).real()
1/2
sage: K.<a> = NumberField(x^2 + x + 1)
sage: a.real()
-1/2
sage: parent(a.real())
Rational Field
sage: K.<i> = QuadraticField(-1)
sage: i.real()
0

round()¶

Returns the round (nearest integer).

EXAMPLES:

sage: K.<sqrt7> = QuadraticField(7, name='sqrt7')
sage: sqrt7.round()
3
sage: (-sqrt7).round()
-3
sage: (12/313*sqrt7 - 1745917/2902921).round()
0
sage: (12/313*sqrt7 - 1745918/2902921).round()
-1

sign()¶

Returns the sign of self (0 if zero, +1 if positive and -1 if negative).

EXAMPLES:

sage: K.<sqrt2> = QuadraticField(2, name='sqrt2')
sage: K(0).sign()
0
sage: sqrt2.sign()
1
sage: (sqrt2+1).sign()
1
sage: (sqrt2-1).sign()
1
sage: (sqrt2-2).sign()
-1
sage: (-sqrt2).sign()
-1
sage: (-sqrt2+1).sign()
-1
sage: (-sqrt2+2).sign()
1

sage: K.<a> = QuadraticField(2, embedding=-1.4142)
sage: K(0).sign()
0
sage: a.sign()
-1
sage: (a+1).sign()
-1
sage: (a+2).sign()
1
sage: (a-1).sign()
-1
sage: (-a).sign()
1
sage: (-a-1).sign()
1
sage: (-a-2).sign()
-1

sage: K.<b> = NumberField(x^2 + 2*x + 7, 'b', embedding=CC(-1,-sqrt(6)))
sage: b.sign()
Traceback (most recent call last):
...
ValueError: a complex number has no sign!
sage: K(1).sign()
1
sage: K(0).sign()
0
sage: K(-2/3).sign()
-1

trace()¶

EXAMPLES:

sage: K.<a> = NumberField(x^2+x+41)
sage: a.trace()
-1
sage: a.matrix()
[  0   1]
[-41  -1]

The trace is additive:

sage: K.<a> = NumberField(x^2+7)
sage: (a+1).trace()
2
sage: K(3).trace()
6
sage: (a+4).trace()
8
sage: (a/3+1).trace()
2

class sage.rings.number_field.number_field_element_quadratic.OrderElement_quadratic¶

Bases: sage.rings.number_field.number_field_element_quadratic.NumberFieldElement_quadratic

Element of an order in a quadratic field.

EXAMPLES:

sage: K.<a> = NumberField(x^2 + 1)
sage: O2 = K.order(2*a)
sage: w = O2.1; w
2*a
sage: parent(w)
Order in Number Field in a with defining polynomial x^2 + 1

charpoly(var='x')¶

The characteristic polynomial of this element, which is over \(\ZZ\) because this element is an algebraic integer.

EXAMPLES:

sage: K.<a> = NumberField(x^2 - 5)
sage: R = K.ring_of_integers()
sage: b = R((5+a)/2)
sage: f = b.charpoly('x'); f
x^2 - 5*x + 5
sage: f.parent()
Univariate Polynomial Ring in x over Integer Ring
sage: f(b)
0

inverse_mod(I)¶

Return an inverse of self modulo the given ideal.

INPUT:

I - may be an ideal of self.parent(), or an element or list of elements of self.parent() generating a nonzero ideal. A ValueError is raised if I is non-integral or is zero. A ZeroDivisionError is raised if I + (x) != (1).

EXAMPLES:

sage: OE.<w> = EquationOrder(x^2 - x + 2)
sage: w.inverse_mod(13) == 6*w - 6
True
sage: w*(6*w - 6) - 1
-13
sage: w.inverse_mod(13).parent() == OE
True
sage: w.inverse_mod(2*OE)
Traceback (most recent call last):
...
ZeroDivisionError: w is not invertible modulo Fractional ideal (2)

minpoly(var='x')¶

The minimal polynomial of this element over \(\ZZ\).

EXAMPLES:

sage: K.<a> = NumberField(x^2 + 163)
sage: R = K.ring_of_integers()
sage: f = R(a).minpoly('x'); f
x^2 + 163
sage: f.parent()
Univariate Polynomial Ring in x over Integer Ring
sage: R(5).minpoly()
x - 5

norm()¶

The norm of an element of the ring of integers is an Integer.

EXAMPLES:

sage: K.<a> = NumberField(x^2 + 3)
sage: O2 = K.order(2*a)
sage: w = O2.gen(1); w
2*a
sage: w.norm()
12
sage: parent(w.norm())
Integer Ring

trace()¶

The trace of an element of the ring of integers is an Integer.

EXAMPLES:

sage: K.<a> = NumberField(x^2 - 5)
sage: R = K.ring_of_integers()
sage: b = R((1+a)/2)
sage: b.trace()
1
sage: parent(b.trace())
Integer Ring

class sage.rings.number_field.number_field_element_quadratic.Q_to_quadratic_field_element¶

Bases: sage.categories.morphism.Morphism

Morphism that coerces from rationals to elements of a quadratic number field K.

EXAMPLES:

sage: K.<a> = QuadraticField(-3)
sage: f = K.coerce_map_from(QQ); f
Natural morphism:
  From: Rational Field
  To:   Number Field in a with defining polynomial x^2 + 3 with a = 1.732050807568878?*I
sage: f(3/1)
3
sage: f(1/2).parent() is K
True

class sage.rings.number_field.number_field_element_quadratic.Z_to_quadratic_field_element¶

Bases: sage.categories.morphism.Morphism

Morphism that coerces from integers to elements of a quadratic number field K.

EXAMPLES:

sage: K.<a> = QuadraticField(3)
sage: phi = K.coerce_map_from(ZZ); phi
Natural morphism:
  From: Integer Ring
  To:   Number Field in a with defining polynomial x^2 - 3 with a = 1.732050807568878?
sage: phi(4)
4
sage: phi(5).parent() is K
True

Optimized Quadratic Number Field Elements¶

Previous topic

Next topic

This Page