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Wednesday, June 5, 2013

Generalization of the Primary Decomposition Theorem (10.3.22)

Dummit and Foote Abstract Algebra, section 10.3, exercise 22:

MathJax TeX Test Page Let M=Tor(M) be an R-module for R a PID and let pR be prime. The p-primary component of M is the set of all elements of M that are annihilated by some positive power of p.

(a) Prove the p-primary component of M is a submodule of M.
(b) Prove this definition of p-primary components of M is consistent with the one given in 10.3.18 when Ann(M)0.
(c) Letting Mp be the p-primary component of M, proveM=pprimeMpProof: (a) Let x,yMp with pax=pby=0. We have pa+b(x+ry)=pb(pax)+par(pby)=0 so that x+ryMp.

(b) For Ann(M)=(a) for a=pα11...pαkk with the previous definition of Mpi={m | pαiim=0} we clearly have MpiMpi. As well, for any mMpi we have am=0, so that for Ann(m)=(d) we have da. As well, by mMpi we have dpai for some positive a. This implies d(a,pai)=pbi for bαi so that now pαiim=pc(dm)=0 and Mpi=Mpi.

(c) Choose arbitrary xM. Letting X=Rx we have (a)Ann(x)=(b) so that b|a=pα11...pαkk. Say b=qβ11...qβkk where βiαi. By 10.3.18 we have X=Xq1...XqkMp1...Mpk so that x is writable as a sum while in Mp1+...+Mpk and now M=pprimeMp.

Proceed in the fashion of 10.3.22(ii) (i). Let mMp1(Mp2+...+Mpk) for any selection such that pj are distinct primes. Write m=n1=n2+...+nk in accordance with the first and second halves, with pαiini=0 for any i. For Ann(m)=(d) have dpα11 for some positive a, and by observing the result of (2ikpαkk)m, we notice d2ikpαkk and now d=1 and 1m=m=0. 

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