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In abstract algebra, the direct sum is a construction which combines several modules into a new, larger module. The direct sum of modules is the smallest module which contains the given modules as submodules with no "unnecessary" constraints, making it an example of a coproduct. Contrast with the direct product, which is the dual notion.
For example, the coproduct in the category of groups, called the free product, is quite complicated. On the other hand, in the category of abelian groups (and equally for vector spaces), the coproduct, called the direct sum, consists of the elements of the direct product which have only finitely many nonzero terms. (It therefore coincides ...
A decomposition with local endomorphism rings [5] (cf. #Azumaya's theorem): a direct sum of modules whose endomorphism rings are local rings (a ring is local if for each element x, either x or 1 − x is a unit). Serial decomposition: a direct sum of uniserial modules (a module is uniserial if the lattice of submodules is a finite chain [6]).
Every vector space is a free module, [1] but, if the ring of the coefficients is not a division ring (not a field in the commutative case), then there exist non-free modules. Given any set S and ring R, there is a free R-module with basis S, which is called the free module on S or module of formal R-linear combinations of the elements of S.
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 strictly smaller when the index set is infinite, because an element of the direct product can have infinitely many nonzero coordinates.
A pair of Saturday NFL games drew a larger viewing audience than college football for the rollout of the sport's 12-team playoff. The playoff game between SMU and Penn State averaged 6.4 million ...
The biproduct is again the direct sum, and the zero object is the trivial vector space. More generally, biproducts exist in the category of modules over a ring. On the other hand, biproducts do not exist in the category of groups. [4] Here, the product is the direct product, but the coproduct is the free product.
Week 9 has come and gone. Time to set our sights for Week 10. Matt Harmon and Sal Vetri are back for another 'Data Dump Wednesday' by sharing 10 data points you need to know for Week 10 to ...