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单词 AlgebraicCategoriesWithoutFreeObjects
释义

algebraic categories without free objects


An initial objectMathworldPlanetmath is always a free object. So in the context of algebraicsystems with a trivial object, such as groups, or modules, there is alwaysat least one free object. However, we usually dismiss this example as itdoes not lead to any useful results such as the existence of presentationsMathworldPlanetmathPlanetmathPlanetmath.

However, there are many ways in which a cateogry of algebraic objects canfail to include non-trivial free objects.

1 Restriction to finite sets

The restrictionPlanetmathPlanetmathPlanetmath of a categoryMathworldPlanetmath which naturally includes infiniteMathworldPlanetmathPlanetmath objectscan often be restricted to just the finite objects and in so doing oftenremove all non-trivial free objects.

  • The category of finite groupsMathworldPlanetmath has only the trivial free object. Indeed,even the rank 1 free groupMathworldPlanetmath, the integers is already infinite.

  • Similarly, finite modules of in a module category over an infinitering are never free. For examples use the rings , m[x],etc.

However, this is not always the case. For example, if we consider finitep-modules (vector spaces) each of these are free.

2 Homomorphism restrictions

In the category of rings with 1 it is often beneficial to force all ringhomomorphismsMathworldPlanetmath to be unital. However, this restriction can preventthe construction of free objects.

Suppose F is a free ring in the category of rings with positive characteristic. Then we ask, what is the characteristicPlanetmathPlanetmath of F?

If it is m>0 then we choose another ring R of a different characteristic,a characteristic relatively prime to m, and then there can be nounital homomorphismMathworldPlanetmathPlanetmathPlanetmathPlanetmathPlanetmathPlanetmathPlanetmathPlanetmath from F to R. So F must have characteristic 0.In contrast to the above examples we have not excluded infinite objectsin this restriction. This example is even more powerful than thoseabove as it also exclude the existance of an initial object, so indeedNO free objects exist in this category.

If we return to the full category of unital rings we observeevery ring is a -algebraMathworldPlanetmathPlanetmath we can use the free associativealgebras X does exist here.

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更新时间:2025/5/4 4:19:31