*****
Rings
*****

.. index::
   pair: matrix; ring

.. _section_matrix-ring:

Matrix rings
============
How do you construct a matrix ring over a finite ring in Sage? The
``MatrixSpace`` constructor accepts any ring as a base ring. Here's
an example of the syntax:

::

    sage: R = IntegerModRing(51)
    sage: M = MatrixSpace(R,3,3)
    sage: M(0)
    [0 0 0]
    [0 0 0]
    [0 0 0]
    sage: M(1)
    [1 0 0]
    [0 1 0]
    [0 0 1]
    sage: 5*M(1)
    [5 0 0]
    [0 5 0]
    [0 0 5]

.. index::
   pair: polynomial; ring

.. _section-polynomial-ring:

Polynomial rings
================

How do you construct a polynomial ring over a finite field in Sage?
Here's an example:

::

    sage: R = PolynomialRing(GF(97),'x')
    sage: x = R.gen()
    sage: f = x^2+7
    sage: f in R
    True

Here's an example using the Singular interface:

::

    sage: R = singular.ring(97, '(a,b,c,d)', 'lp')
    sage: I = singular.ideal(['a+b+c+d', 'ab+ad+bc+cd', 'abc+abd+acd+bcd', 'abcd-1'])
    sage: R
    //   characteristic : 97
    //   number of vars : 4
    //        block   1 : ordering lp
    //                  : names    a b c d
    //        block   2 : ordering C
    sage: I
    a+b+c+d,
    a*b+a*d+b*c+c*d,
    a*b*c+a*b*d+a*c*d+b*c*d,
    a*b*c*d-1

Here is another approach using GAP:

::

    sage: R = gap.new("PolynomialRing(GF(97), 4)"); R
    PolynomialRing( GF(97), ["x_1", "x_2", "x_3", "x_4"] )
    sage: I = R.IndeterminatesOfPolynomialRing(); I
    [ x_1, x_2, x_3, x_4 ]
    sage: vars = (I.name(), I.name(), I.name(), I.name())
    sage: _ = gap.eval("x_0 := %s[1];; x_1 := %s[2];; x_2 := %s[3];;\
    ...   x_3 := %s[4];;"%vars)
    sage: f = gap.new("x_1*x_2+x_3"); f
    x_2*x_3+x_4
    sage: f.Value(I,[1,1,1,1])
    Z(97)^34

.. index:: p-adics

.. _section-padics:

:math:`p`-adic numbers
========================

How do you construct :math:`p`-adics in Sage? A great deal of
progress has been made on this (see SageDays talks by David Harvey
and David Roe). Here only a few of the simplest examples are
given.

To compute the characteristic and residue class field of the ring
``Zp`` of integers of ``Qp``, use the syntax illustrated by the
folowing examples.

::

    sage: K = Qp(3)
    sage: K.residue_class_field()
    Finite Field of size 3
    sage: K.residue_characteristic()
    3
    sage: a = K(1); a
    1 + O(3^20)
    sage: 82*a
    1 + 3^4 + O(3^20)
    sage: 12*a
    3 + 3^2 + O(3^21)
    sage: a in K
    True
    sage: b = 82*a
    sage: b^4
    1 + 3^4 + 3^5 + 2*3^9 + 3^12 + 3^13 + 3^16 + O(3^20)

.. index::
   pair: polynomial; quotient ring

Quotient rings of polynomials
=============================

How do you construct a quotient ring in Sage?

We create the quotient ring :math:`GF(97)[x]/(x^3+7)`, and
demonstrate many basic functions with it.

::

    sage: R = PolynomialRing(GF(97),'x')
    sage: x = R.gen()
    sage: S = R.quotient(x^3 + 7, 'a')
    sage: a = S.gen()
    sage: S
    Univariate Quotient Polynomial Ring in a over Finite Field of size 97 with 
    modulus x^3 + 7
    sage: S.is_field()
    True
    sage: a in S
    True
    sage: x in S
    True
    sage: S.polynomial_ring()
    Univariate Polynomial Ring in x over Finite Field of size 97
    sage: S.modulus()
    x^3 + 7
    sage: S.degree()
    3

In Sage, ``in`` means that there is a "canonical coercion" into the
ring. So the integer :math:`x` and :math:`a` are both in
:math:`S`, although :math:`x` really needs to be coerced.

You can also compute in quotient rings without actually computing
then using the command ``quo_rem`` as follows.

::

    sage: R = PolynomialRing(GF(97),'x')
    sage: x = R.gen()
    sage: f = x^7+1
    sage: (f^3).quo_rem(x^7-1)
    (x^14 + 4*x^7 + 7, 8)