Anonymous asked in Science & MathematicsMathematics · 9 years ago

Linear algebra proof help?

Suppose that V is finite dimensional and S,T are elements of L(V). Prove that ST=I if and only if TS=I.

My teacher suggested we use the rank + nullity theorem to prove this, but I have no idea how.

1 Answer

  • 9 years ago
    Favorite Answer

    I hope this hepls:

    if ST=I then

    note that (TS)^2=TSTS=TS, this is idempotent.

    Then TS(TS-I)=0 therefore STS(TS-I)=S0, S(TS-I)=0.

    The kernel of S is the rank of (TS-I). If S has a non trivial kernel then there is v non zero in V that Sv=0.

    If Tw=v then STw=0 or if no point of w in V is such that Tw=v then STw is not v, not even if w=v; So ST cannot by the identity.

    Hence S most have a trivial kernel this TS=I.

    • Login to reply the answers
Still have questions? Get your answers by asking now.