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Sage

We have already seen some integral domains and unique factorizations in the previous two chapters.

Sage

We have already seen some integral domains and unique factorizations in the previous two chapters. In addition to what we have already seen, Sage has support for some of the topics from this section, but the coverage is limited. Some functions will work for some rings and not others, while some functions are not yet part of Sage. So we will give some examples, but this is far from comprehensive.

Field of Fractions

Sage is frequently able to construct a field of fractions, or identify a certain field as the field of fractions. For example, the ring of integers and the field of rational numbers are both implemented in Sage, and the integers know that the rationals is it's field of fractions.

In other cases Sage will construct a fraction field, in the spirit of . So it is then possible to do basic calculations in the constructed field.

Prime Subfields

says every field of characteristic \(p\) has a subfield isomorphic to \({\mathbb Z}_p\). For a finite field, the exact nature of this subfield is not a surprise, but Sage will allow us to extract it easily.

More generally, the fields mentioned in the conclusions of and are known as the prime subfield of the ring containing them. Here is an example of the characteristic zero case.

In a rough sense, every characteristic zero field contains a copy of the rational numbers (the fraction field of the integers), which can explain Sage's extensive support for rings and fields that extend the integers and the rationals.

Integral Domains

Sage can determine if some rings are integral domains and we can test products in them. However, notions of units, irreducibles or prime elements are not generally supported (outside of what we have seen for polynomials in the previous chapter). Worse, the construction below creates a ring within a larger field and so some functions (such as .is_unit()) pass through and give misleading results. This is because the construction below creates a ring known as an order in a number field.

The following is a bit misleading, since \(4\), as an element of \({\mathbb Z}[\sqrt{3}i]\) does not have a multiplicative inverse, though seemingly we can compute one.

Principal Ideals

When a ring is a principal ideal domain, such as the integers, or polynomials over a field, Sage works well. Beyond that, support begins to weaken.

Symbols used here

\sqrt{x},\ \sqrt[n]{x}
square root, n-th root
The non-negative number whose square (n-th power) is x.
i
imaginary unit
i² = −1.
\mathbb{N},\ \mathbb{Z},\ \mathbb{Q},\ \mathbb{R},\ \mathbb{C}
number sets
Naturals, integers, rationals, reals, complex numbers.
x \in A,\ A \subseteq B
element of, subset
x belongs to A; every element of A is in B.
\blacksquare\ \text{or}\ \square
end of proof (halmos)
Marks the point where the statement has been established.
a \equiv b \pmod n
congruent modulo n
n divides a − b; a and b have the same remainder.
a \mid b,\ \gcd(a,b)
divides, greatest common divisor
b is a multiple of a; the largest number dividing both.
(G, \cdot),\ e,\ g^{-1}
group, identity, inverse
A set with an operation; the do-nothing element; the element that undoes g.
G \cong H,\ G / N
isomorphic, quotient group
Same structure; the group of cosets of a normal subgroup N.
\mathbb{Z}/n\mathbb{Z},\ \mathbb{Z}_n
integers modulo n
The remainders 0…n−1 with clock arithmetic.
\operatorname{Hom}(A, B),\ f \circ g
arrows from A to B, composition
The set of morphisms; do g then f.

Questions people ask

What is a group, in plain words?

A set with one operation that is associative, has an identity, and lets every element be undone. Symmetries of any object form a group — that is where the idea came from.

What is the difference between a ring and a field?

A ring has addition and multiplication that behave like the integers (you cannot always divide); a field is a ring where every non-zero element has a reciprocal, like the rationals or the reals.

Andramo ny anao manokana

Parts of this page are adapted from Judson, Abstract Algebra: Theory and Applications (GFDL 1.3). Condensed and re-explained here; errors are ours.

Mbola maro ao Abstract Algebra

GroupsSubgroups, cosets and Lagrange's theoremCyclic groups and permutation groupsHomomorphisms, normal subgroups and quotient groupsRings and fieldsGalois theory: why the quintic has no formula