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单词 FinitelyGeneratedModule
释义

finitely generated module


A module X over a ring R is said to be finitely generatedMathworldPlanetmathPlanetmath if there is a finite subset Y of X such that Y spans X. Let us recall that the span of a (not necessarily finite) set X of vectors is the class of all (finite) linear combinationsMathworldPlanetmath of elements of S; moreover, let us recall that the span of the empty setMathworldPlanetmath is defined to be the singleton consisting of only one vector, the zero vectorMathworldPlanetmath 0. A module X is then called cyclic if it can be a singleton.

Examples. Let R be a commutative ring with 1 and x be an indeterminate.

  1. 1.

    Rx={rxrR} is a cyclic R-module generated by {x}.

  2. 2.

    RRx is a finitely-generated R-module generated by {1,x}. Any element in RRxcan be expressed uniquely as r+sx.

  3. 3.

    R[x] is not finitely generated as an R-module. For if there is a finite setMathworldPlanetmath Y R[x], taking d to be the largest of all degrees of polynomials in Y, then xd+1 would not be in the of Y, assumed to be R[x], which is a contradictionMathworldPlanetmathPlanetmath. (Note, however, that R[x] is finitely-generated as an R-algebra.)

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更新时间:2025/5/5 2:55:20