Abelian variety

In abstract algebra, a free abelian group or free Z-module is an abelian group with a basis. That is, it is a set together with an associative, commutative, and invertible binary operation, and its basis is a subset of its elements such that every element of the group can be written in one and only one way as a linear combination of basis elements with integer coefficients, finitely many of which are nonzero.  The elements of a free abelian group with basis B are also known as formal sums over B.  Informally, formal sums may also be seen as signed multisets with elements in B. Free abelian groups and formal sums have applications in algebraic topology, where they are used to define chain groups, and in algebraic geometry, where they are used to define divisors.

Every set B has a unique free abelian group with B as its basis. This group may be constructed as a direct sum of copies of the additive group of the integers, with one copy per member of B. Alternatively, it may be constructed as the set of functions from B to the integers that have finitely many nonzero values, with pointwise addition, or by a presentation with the elements of B as its generators and with the commutators of pairs of members as its relators.

Free abelian groups have properties which make them similar to vector spaces and allow a general abelian group to be understood as a quotient of a free abelian group by "relations".  Every free abelian group has a rank defined as the cardinality of a basis.  The rank determines the group up to isomorphism, and the elements of such a group can be written as finite formal sums of the basis elements.  Every subgroup of a free abelian group is itself free abelian, which allows the description of a general abelian group as a cokernel of an injective homomorphism between free abelian groups.

Examples and constructions

Integers and lattices

The integers, under the addition operation, form a free abelian group with the basis {1}. Every integer n is a linear combination of basis elements with integer coefficients: namely, n = n × 1, with the coefficient n.

The two-dimensional integer lattice, consisting of the points in the plane with integer Cartesian coordinates, forms a free abelian group under vector addition with the basis {(0,1), (1,0)}. If we say $\ e_{1}=(1,0)$ and $\ e_{2}=(0,1)$, then the element (4,3) can be written

$\ (4,3)=4e_{1}+3e_{2}$ where 'multiplication' is defined so that $\ 4e_{1}:=e_{1}+e_{1}+e_{1}+e_{1}$.

In this basis, there is no other way to write (4,3), but with a different basis such as {(1,0),(1,1)}, where $\ f_{1}=(1,0)$ and $\ f_{2}=(1,1)$, it can be written as

$\ (4,3)=f_{1}+3f_{2}$.

More generally, every lattice forms a finitely-generated free abelian group. The d-dimensional integer lattice has a natural basis consisting of the positive integer unit vectors, but it has many other bases as well: if M is a d × d integer matrix with determinant ±1, then the rows of M form a basis, and conversely every basis of the integer lattice has this form. For more on the two-dimensional case, see fundamental pair of periods.

Direct sums, direct products, and trivial group

The direct product of two free abelian groups is itself free abelian, with basis the disjoint union of the bases of the two groups. More generally the direct product of any finite number of free abelian groups is free abelian. The d-dimensional integer lattice, for instance, is isomorphic to the direct product of d copies of the integer group Z.

The trivial group {0} is also considered to be free abelian, with basis the empty set. It may be interpreted as a direct product of zero copies of Z.

For infinite families of free abelian groups, the direct product (the family of tuples of elements from each group, with pointwise addition) is not necessarily free abelian. For instance the Baer–Specker group $\mathbb {Z} ^{\mathbb {N} }$, an uncountable group formed as the direct product of countably many copies of $\mathbb {Z}$, was shown in 1937 by Reinhold Baer to not be free abelian; Ernst Specker proved in 1950 that every countable subgroup of $\mathbb {Z} ^{\mathbb {N} }$ is free abelian. The direct sum of finitely many groups is the same as the direct product, but differs from the direct product on an infinite number of summands; its elements consist of tuples of elements from each group with all but finitely many of them equal to the identity element. As in the case of a finite number of summands, the direct sum of infinitely many free abelian groups remains free abelian, with a basis formed by (the images of) a disjoint union of the bases of the summands.

The tensor product of two free abelian groups is always free abelian, with a basis that is the Cartesian product of the bases for the two groups in the product.

Every free abelian group may be described as a direct sum of copies of $\mathbb {Z}$, with one copy for each member of its basis. This construction allows any set B to become the basis of a free abelian group.

Integer functions and formal sums

Given a set B, one can define a group $\mathbb {Z} ^{(B)}$ whose elements are functions from B to the integers, where the parenthesis in the superscript indicates that only the functions with finitely many nonzero values are included. If f(x) and g(x) are two such functions, then f + g is the function whose values are sums of the values in f and g: that is, (f + g)(x) = f(x) + g(x) . This pointwise addition operation gives $\mathbb {Z} ^{(B)}$ the structure of an abelian group.

Each element x from the given set B corresponds to a member of $\mathbb {Z} ^{(B)}$, the function e_x for which e_x(x) = 1 and for which e_x(y) = 0 for all y ≠ x. Every function f in $\mathbb {Z} ^{(B)}$ is uniquely a linear combination of a finite number of basis elements:

$f=\sum _{\{x\mid f(x)\neq 0\}}f(x)e_{x}$

Thus, these elements e_x form a basis for $\mathbb {Z} ^{(B)}$, and $\mathbb {Z} ^{(B)}$ is a free abelian group. In this way, every set B can be made into the basis of a free abelian group.

The free abelian group with basis B is unique up to isomorphism, and its elements are known as formal sums of elements of B. They may also be interpreted as the signed multisets of finitely many elements of B. For instance, in algebraic topology, chains are formal sums of simplices, and the chain group is the free abelian group whose elements are chains. In algebraic geometry, the divisors of a Riemann surface (a combinatorial description of the zeros and poles of meromorphic functions) form an uncountable free abelian group, consisting of the formal sums of points from the surface.

Presentation

The free abelian group with basis B has a presentation in which the generators are the elements of B, and the relators are the commutators of pairs of elements of B.

This fact, together with the fact that every subgroup of a free abelian group is free abelian (below) can be used to show that every finitely generated abelian group is finitely presented. For, if G is finitely generated by a set B, it is a quotient of the free abelian group over B by a free abelian subgroup, the subgroup generated by the relators of the presentation of G. But since this subgroup is itself free abelian, it is also finitely generated, and its basis (together with the commutators over B) forms a finite set of relators for a presentation of G.

Terminology

Every abelian group may be considered as a module over the integers by considering the scalar multiplication of a group member by an integer defined as follows:

${\begin{aligned}0\,x&=0\\1\,x&=x\\n\,x&=x+(n-1)\,x\qquad {\text{if}}\quad n>1\\(-n)\,x&=-(n\,x)\qquad {\text{if}}\quad n<0\end{aligned}}$

A free module is a module that can be represented as a direct sum over its base ring, so free abelian groups and free $\mathbb {Z}$-modules are equivalent concepts: each free abelian group is (with the multiplication operation above) a free $\mathbb {Z}$-module, and each free $\mathbb {Z}$-module comes from a free abelian group in this way.

Unlike vector spaces, not all abelian groups have a basis, hence the special name for those that do. For instance, any torsion $\mathbb {Z}$-module, and thus any finite abelian group, is not a free abelian group, because 0 may be decomposed in several ways on any set of elements which could be a candidate for a basis: $0=0\,b=n\,b$ for some positive integer n. On the other hand, many important property of free abelian groups may be generalized to free modules over a principal ideal domain.

Note that a free abelian group is not a free group except in two cases: a free abelian group having an empty basis (rank 0, giving the trivial group) or having just 1 element in the basis (rank 1, giving the infinite cyclic group).  Other abelian groups are not free groups because in free groups ab must be different from ba if a and b are different elements of the basis, while in free abelian groups they must be identical.  Free groups are the free objects in the category of groups, that is, the "most general" or "least constrained" groups with a given number of generators, whereas free abelian groups are the free objects in the category of abelian groups; in the general category of groups, it is an added constraint to demand that ab = ba, whereas this is a necessary property in the category of abelian groups.

Properties

Universal property

If F is a free abelian group with basis B, then we have the following universal property: for every arbitrary function f from B to some abelian group A, there exists a unique group homomorphism from F to A which extends f.  By a general property of universal properties, this shows that "the" abelian group of base B is unique up to an isomorphism. This allows to use this universal property as a definition of the free abelian group of base B and shows that all the other definitions are equivalent.

Rank

Every two bases of the same free abelian group have the same cardinality, so the cardinality of a basis forms an invariant of the group known as its rank. In particular, a free abelian group is finitely generated if and only if its rank is a finite number n, in which case the group is isomorphic to $\mathbb {Z} ^{n}$.

The rank of an abelian group G is defined as the rank of a free abelian subgroup F of G for which the quotient group G/F is a torsion group. Equivalently, it is the cardinality of a maximal subset of G that generates a free subgroup. Again, this is a group invariant; it does not depend on the choice of the subgroup.

Subgroups

Every subgroup of a free abelian group is itself a free abelian group.  This result of Richard Dedekind was a precursor to the analogous Nielsen–Schreier theorem that every subgroup of a free group is free, and is a generalization of the fact that every nontrivial subgroup of the infinite cyclic group is infinite cyclic.

Theorem: Let $F$ be a free abelian group and let $G\subset F$ be a subgroup.  Then $G$ is a free abelian group.

The proof needs the axiom of choice. A proof using Zorn's lemma (one of many equivalent assumptions to the axiom of choice) can be found in Serge Lang's Algebra. Solomon Lefschetz and Irving Kaplansky have claimed that using the well-ordering principle in place of Zorn's lemma leads to a more intuitive proof.

In the case of finitely generated free groups, the proof is easier, and leads to a more precise result.

Theorem: Let $G$ be a subgroup of a finitely generated free abelian group $F$. Then $G$ is free and there exists a basis $(e_{1},\ldots ,e_{n})$ of $F$ and positive integers $d_{1}|d_{2}|\ldots |d_{k}$ (that is, each one divides the next one) such that $(d_{1}e_{1},\ldots ,d_{k}e_{k})$ is a basis of $G.$ Moreover, the sequence $d_{1},d_{2},\ldots ,d_{k}$ depends only on $F$ and $G$ and not on the particular basis $(e_{1},\ldots ,e_{n})$ that solves the problem.

A constructive proof of the existence part of the theorem is provided by any algorithm computing the Smith normal form of a matrix of integers. Uniqueness follows from the fact that, for any r ≤ k, the greatest common divisor of the minors of rank r of the matrix is not changed during the Smith normal form computation and is the product $d_{1}\cdots d_{r}$ at the end of the computation.

Torsion and divisibility

All free abelian groups are torsion-free, meaning that there is no group element x and nonzero integer n such that nx = 0. Conversely, all finitely generated torsion-free abelian groups are free abelian. The same applies to flatness, since an abelian group is torsion-free if and only if it is flat.

The additive group of rational numbers Q provides an example of a torsion-free (but not finitely generated) abelian group that is not free abelian. One reason that Q is not free abelian is that it is divisible, meaning that, for every element x of Q and every nonzero integer n, it is possible to express x as a scalar multiple ny of another element y. In contrast, non-zero free abelian groups are never divisible, because it is impossible for any of their basis elements to be nontrivial integer multiples of other elements.

Relation to arbitrary abelian groups

Given an arbitrary abelian group A, there always exists a free abelian group F and a surjective group homomorphism from F to A. One way of constructing a surjection onto a given group A is to let $F=\mathbb {Z} ^{(A)}$ be the free abelian group over A, represented as the set of functions from A to the integers with finitely many nonzeros. Then a surjection can be defined from the representation of members of F as formal sums of members of A:

$f=\sum _{\{x\mid f(x)\neq 0\}}f(x)e_{x}\mapsto \sum _{\{x\mid f(x)\neq 0\}}f(x)x,$

where the first sum is in F and the second sum is in A. This construction can be seen as an instance of the universal property: this surjection is the unique group homomorphism which extends the function $e_{x}\mapsto x$.

When F and A are as above, the kernel G of the surjection from F to A is also free abelian, as it is a subgroup of F (the subgroup of elements mapped to the identity). Therefore, these groups form a short exact sequence

0 → G → F → A → 0

in which F and G are both free abelian and A is isomorphic to the factor group F/G.  This is a free resolution of A.  Furthermore, assuming the axiom of choice, the free abelian groups are precisely the projective objects in the category of abelian groups.

References

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