||It has been suggested that this article be merged into direct sum of modules. (Discuss) Proposed since January 2014.|
The direct sum is an operation from abstract algebra, a branch of mathematics. As an example, consider the direct sum , where is the set of real numbers. is the Cartesian plane, the xy-plane from elementary algebra. In general, the direct sum of two objects is another object of the same type, so the direct sum of two geometric objects is a geometric object and the direct sum of two sets is a set.
To see how direct sum is used in abstract algebra, consider the most elementary structure in abstract algebra, the abelian group. The direct sum of two abelian groups and is another abelian group consisting of the ordered pairs where and . To add ordered pairs, we define the sum to be ; in other words addition is defined coordinate-wise. A similar process can be used to form the direct sum of any two algebraic structures, such as rings, modules, and vector spaces.
We can also form direct sums with any number of summands, for example , provided and are the same kinds of algebraic structures, that is, all groups or all rings or all vector spaces.
In the case of two summands, or any finite number of summands, the direct sum is the same as the direct product. If the arithmetic operation is written as +, as it usually is in abelian groups, then we use the direct sum. If the arithmetic operation is written as × or ⋅ or using juxtaposition (as in the expression ) we use direct product.
In the case where infinitely many objects are combined, most authors make a distinction between direct sum and direct product. As an example, consider the direct sum and direct product of infinitely many real lines. An element in the direct sum is an infinite sequence, such as (1,2,3,...) but in the direct sum, there would be a requirement that all but finitely many coordinates be zero, so the sequence (1,2,3,...) would be an element of the direct product but not of the direct sum, while (1,2,0,0,0,...) would be an element of both. More generally, if a + sign is used, all but finitely many coordinates must be zero, while if some form of multiplication is used, all but finitely many coordinates must be 1. In more technical language, if the summands are , the direct sum is defined to be the set of tuples with such that for all but finitely many i. The direct sum is contained in the direct product , but is usually strictly smaller when the index set is infinite, because direct products do not have the restriction that all but finitely many coordinates must be zero.
For example, the xy-plane, a two-dimensional vector space, can be thought of as the direct sum of two one-dimensional vector spaces, namely the x and y axes. In this direct sum, the x and y axes intersect only at the origin (the zero vector). Addition is defined coordinate-wise, that is , which is the same as vector addition.
Given two objects and , their direct sum is written as . Given an indexed family of objects , indexed with , the direct sum may be written . Each Ai is called a direct summand of A. If the index set is finite, the direct sum is the same as the direct product. In the case of groups, if the group operation is written as the phrase "direct sum" is used, while if the group operation is written the phrase "direct product" is used. When the index set is infinite, the direct sum is not the same as the direct product. In the direct sum, all but finitely many coordinates must be zero.
Internal and external direct sums
A distinction is made between internal and external direct sums, though the two are isomorphic. If the factors are defined first, and then the direct sum is defined in terms of the factors, we have an external direct sum. For example, if we define the real numbers and then define the direct sum is said to be external. If, on the other hand, we first define some set, and then write as the direct sum of two of its proper subsets, then the direct sum is said to be internal. For an example of an internal direct sum, consider , the integers modulo six, whose elements are . .
Types of direct sum
Direct sum of abelian groups
The direct sum of abelian groups is a prototypical example of a direct sum. Given two abelian groups and , their direct sum is the same as their direct product, that is the underlying set is the Cartesian product and the group operation is defined component-wise:
This definition generalizes to direct sums of finitely many abelian groups.
For an infinite family of abelian groups Ai for i ∈ I, the direct sum
Direct sum of modules
The direct sum of modules is a construction which combines several modules into a new module.
Direct sum of group representations
The direct sum of group representations generalizes the direct sum of the underlying modules, adding a group action to it. Specifically, given a group G and two representations V and W of G (or, more generally, two G-modules), the direct sum of the representations is V ⊕ W with the action of g ∈ G given component-wise, i.e.
- g·(v, w) = (g·v, g·w).
Direct sum of rings
Some authors will speak of the direct sum of two rings when they mean the direct product , but this should be avoided since does not receive natural ring homomorphisms from R and S: in particular, the map sending r to (r,0) is not a ring homomorphism since it fails to send 1 to (1,1) (assuming that 0≠1 in S). Thus is not a coproduct in the category of rings, and should not be written as a direct sum. (The coproduct in the category of commutative rings is the tensor product of rings. In the category of rings, the coproduct is given by a construction similar to the free product of groups.)
Use of direct sum terminology and notation is especially problematic when dealing with infinite families of rings: If is an infinite collection of nontrivial rings, then the direct sum of the underlying additive groups can be equipped with termwise multiplication, but this produces a rng, i.e., a ring without a multiplicative identity.
Direct sum in categories
In such a category finite products and coproducts agree and the direct sum is either of them, cf. biproduct.
General case :  In category theory the direct sum is often, but not always, the coproduct in the category of the mathematical objects in question. For example, in the category of abelian groups, direct sum is a coproduct. This is also true in the category of modules.
The direct sum comes equipped with a homomorphism for each j. Given another abelian group B (with the same additional structure) equipped with a homomorphism for every j, there is a unique homomorphism (called the sum of the gj) such that for all j. Thus the direct sum is the coproduct in the appropriate category.