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An element in the direct product is an infinite sequence, such as (1,2,3,...) but in the direct sum, there is 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.
for v, v 1, v 2 ∈ V, w, w 1, w 2 ∈ W, and α ∈ K. The resulting vector space is called the direct sum of V and W and is usually denoted by a plus symbol inside a circle: It is customary to write the elements of an ordered sum not as ordered pairs (v, w), but as a sum v + w. The subspace V × {0} of V ⊕ W is isomorphic to V and is often ...
the subgroups H 1 and H 2 have trivial intersection (i.e., having only the identity element of G in common), G = H 1, H 2 ; in other words, G is generated by the subgroups H 1 and H 2. More generally, G is called the direct sum of a finite set of subgroups {H i} if each H i is a normal subgroup of G,
(x 1, y 1) + (x 2, y 2) = (x 1 + x 2, y 1 + y 2). Let R + be the group of positive real numbers under multiplication. Then the direct product R + × R + is the group of all vectors in the first quadrant under the operation of component-wise multiplication (x 1, y 1) × (x 2, y 2) = (x 1 × x 2, y 1 × y 2). Let G and H be cyclic groups with two ...
This applies also when E and F are linear subspaces or submodules of the vector space or module V. 2. Direct sum: if E and F are two abelian groups, vector spaces, or modules, then their direct sum, denoted is an abelian group, vector space, or module (respectively) equipped with two monomorphisms: and : such that is the internal direct sum of ...
The disjoint union space X, together with the canonical injections, can be characterized by the following universal property: If Y is a topological space, and f i : X i → Y is a continuous map for each i ∈ I, then there exists precisely one continuous map f : X → Y such that the following set of diagrams commute:
An abelian category [4] C is called semi-simple if there is a collection of simple objects , i.e., ones with no subobject other than the zero object 0 and itself, such that any object X is the direct sum (i.e., coproduct or, equivalently, product) of finitely many simple objects.
The direct sum and direct product are not isomorphic 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.