 Direct product

In mathematics, one can often define a direct product of objects already known, giving a new one. This is generally the Cartesian product of the underlying sets, together with a suitably defined structure on the product set. More abstractly, one talks about the product in category theory, which formalizes these notions.
Examples are the product of sets (see Cartesian product), groups (described below), the product of rings and of other algebraic structures. The product of topological spaces is another instance.
There is also the direct sum – in some areas this is used interchangeably, in others it is a different concept.
Contents
Examples
 If we think of as the set of real numbers, then the direct product is precisely just the cartesian product, .
 If we think of as the group of real numbers under addition, then the direct product still consists of . The difference between this and the preceding example is that is now a group. We have to also say how to add their elements. This is done by letting (a,b) + (c,d) = (a + c,b + d).
 If we think of as the ring of real numbers, then the direct product again consists of . To make this a ring, we say how their elements are added, (a,b) + (c,d) = (a + c,b + d), and how they are multiplied (a,b)(c,d) = (ac,bd).
 However, if we think of as the field of real numbers, then the direct product does not exist  naively defining in a similar manner to the above examples would not result in a field since the element (1,0) does not have a multiplicative inverse.
In a similar manner, we can talk about the product of more than two objects, e.g. . We can even talk about product of infinitely many objects, e.g. .
Group direct product
Main article: Direct product of groupsIn group theory one can define the direct product of two groups (G, *) and (H, ●), denoted by G × H. For abelian groups which are written additively, it may also be called the direct sum of two groups, denoted by .
It is defined as follows:
 the set of the elements of the new group is the cartesian product of the sets of elements of G and H, that is {(g, h): g in G, h in H};
 on these elements put an operation, defined elementwise:
(g, h) × (g' , h' ) = (g * g' , h ● h' )
(Note the operation * may be the same as ●.)
This construction gives a new group. It has a normal subgroup isomorphic to G (given by the elements of the form (g, 1)), and one isomorphic to H (comprising the elements (1, h)).
The reverse also holds, there is the following recognition theorem: If a group K contains two normal subgroups G and H, such that K= GH and the intersection of G and H contains only the identity, then K is isomorphic to G x H. A relaxation of these conditions, requiring only one subgroup to be normal, gives the semidirect product.
As an example, take as G and H two copies of the unique (up to isomorphisms) group of order 2, C_{2}: say {1, a} and {1, b}. Then C_{2}×C_{2} = {(1,1), (1,b), (a,1), (a,b)}, with the operation element by element. For instance, (1,b)*(a,1) = (1*a, b*1) = (a,b), and (1,b)*(1,b) = (1,b^{2}) = (1,1).
With a direct product, we get some natural group homomorphisms for free: the projection maps
 ,
called the coordinate functions.
Also, every homomorphism f on the direct product is totally determined by its component functions .
For any group (G, *), and any integer n ≥ 0, multiple application of the direct product gives the group of all ntuples G^{n} (for n=0 the trivial group). Examples:
 Z^{n}
 R^{n} (with additional vector space structure this is called Euclidean space, see below)
Direct product of modules
The direct product for modules (not to be confused with the tensor product) is very similar to the one defined for groups above, using the cartesian product with the operation of addition being componentwise, and the scalar multiplication just distributing over all the components. Starting from R we get Euclidean space R^{n}, the prototypical example of a real ndimensional vector space. The direct product of R^{m} and R^{n} is R^{m + n}.
Note that a direct product for a finite index is identical to the direct sum . The direct sum and direct product differ only for infinite indices, where the elements of a direct sum are zero for all but for a finite number of entries. They are dual in the sense of Category Theory: the direct sum is the coproduct, while the direct product is the product.
For example, consider and , the infinite direct product and direct sum of the real numbers. Only sequences with a finite number of nonzero elements are in Y. For example, (1,0,0,0,...) is in Y but (1,1,1,1,...) is not. Both of these sequences are in the direct product X; in fact, Y is a proper subset of X (that is, Y⊂X).
Topological space direct product
The direct product for a collection of topological spaces X_{i} for i in I, some index set, once again makes use of the cartesian product
Defining the topology is a little tricky. For finitely many factors, this is the obvious and natural thing to do: simply take as a basis of open sets to be the collection of all cartesian products of open subsets from each factor:
This topology is called the product topology. For example, directly defining the product topology on R^{2} by the open sets of R (disjoint unions of open intervals), the basis for this topology would consist of all disjoint unions of open rectangles in the plane (as it turns out, it coincides with the usual metric topology).
The product topology for infinite products has a twist, and this has to do with being able to make all the projection maps continuous and to make all functions into the product continuous if and only if all its component functions are continuous (i.e. to satisfy the categorical definition of product: the morphisms here are continuous functions): we take as a basis of open sets to be the collection of all cartesian products of open subsets from each factor, as before, with the proviso that all but finitely many of the open subsets are the entire factor:
The more naturalsounding topology would be, in this case, to take products of infinitely many open subsets as before, and this does yield a somewhat interesting topology, the box topology. However it is not too difficult to find an example of bunch of continuous component functions whose product function is not continuous (see the separate entry box topology for an example and more). The problem which makes the twist necessary is ultimately rooted in the fact that the intersection of open sets is only guaranteed to be open for finitely many sets in the definition of topology.
Products (with the product topology) are nice with respect to preserving properties of their factors; for example, the product of Hausdorff spaces is Hausdorff; the product of connected spaces is connected, and the product of compact spaces is compact. That last one, called Tychonoff's theorem, is yet another equivalence to the axiom of choice.
For more properties and equivalent formulations, see the separate entry product topology.
Direct product of binary relations
On the Cartesian product of two sets with binary relations R and S, define (a, b) T (c, d) as a R c and b S d. If R and S are both reflexive, irreflexive, transitive, symmetric, or antisymmetric, relation T has the same property.^{[1]} Combining properties it follows that this also applies for being a preorder and being an equivalence relation. However, if R and S are total relations, T is in general not.
Categorical product
Main article: Product (category theory)The direct product can be abstracted to an arbitrary category. In a general category, given a collection of objects A_{i} and a collection of morphisms p_{i} from A to A_{i} with i ranging in some index set I, an object A is said to be a categorical product in the category if, for any object B and any collection of morphisms f_{i} from B to A_{i}, there exists a unique morphism f from B to A such that f_{i} = p_{i} f and this object A is unique. This not only works for two factors, but arbitrarily (even infinitely) many.
For groups we similarly define the direct product of a more general, arbitrary collection of groups G_{i} for i in I, I an index set. Denoting the cartesian product of the groups by G we define multiplication on G with the operation of componentwise multiplication; and corresponding to the p_{i} in the definition above are the projection maps
 ,
the functions that take to its ith component g_{i}.
Internal and External direct product
See also: Internal direct sumSome authors draw a distinction between an internal direct product and an external direct product. If and , then we say that X is an internal direct product (of A and B); if A and B are not subobjects, then we say that this is an external direct product.
Metric and norm
A metric on a Cartesian product of metric spaces, and a norm on a direct product of normed vector spaces, can be defined in various ways, see for example pnorm.
See also
 Direct sum
 Cartesian product
 Coproduct
 Free product
 Semidirect product
 ZappaSzep product
 Tensor product of graphs
 Orders on the Cartesian product of totally ordered sets
Notes
References
 Lang, Serge (2002), Algebra, Graduate Texts in Mathematics, 211 (Revised third ed.), New York: SpringerVerlag, ISBN 9780387953854, MR1878556
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