Finitely generated modules over a PID isomorphism

Let $M, N, P$ be finitely generated modules over a PID.

  • Show that if $M⊕M≅N⊕N$, then $M≅N$.
  • Show that if $M⊕P≅N⊕P$, then $M≅N$.


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  • Hey, just so it's clear, in the 9th line of the first page what does it say after the "with"? Also, 6th line of the second page it says "q_i,M arrow q_j,N" right?

  • Dtatdm Dtatdm

    With $q_{i,\cdot}=q_{i',\cdot}$, just that we are pairing off the two copies of each elementary divisor. And in the second page, yes. If we map one elementary divisor of M to a another in N then we map the corresponding paired one in M to the corresponding paired one in N.

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