(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,y∈Mp with pax=pby=0. We have pa+b(x+ry)=pb(pax)+par(pby)=0 so that x+ry∈Mp.
(b) For Ann(M)=(a) for a=pα11...pαkk with the previous definition of M∗pi={m | pαiim=0} we clearly have M∗pi⊆Mpi. As well, for any m∈Mpi we have am=0, so that for Ann(m)=(d) we have d∣a. As well, by m∈Mpi we have d∣pai for some positive a. This implies d∣(a,pai)=pbi for b≤αi so that now pαiim=pc(dm)=0 and Mpi=M∗pi.
(c) Choose arbitrary x∈M. Letting X=Rx we have (a)⊆Ann(x)=(b) so that b|a=pα11...pαkk. Say b=qβ11...qβkk where βi≤αi. By 10.3.18 we have X=Xq1⊕...⊕Xqk⊆Mp1⊕...⊕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 m∈Mp1∩(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 d∣pα11 for some positive a, and by observing the result of (∏2≤i≤kpαkk)m, we notice d∣∏2≤i≤kpαkk and now d=1 and 1m=m=0. ◻
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