hi, i have to find out what is the principal part of the laurent serie of f(z)=z^4/(z^2-1) about infinity.
is z^2 true?
regards
Yes.
Let , let , and get
Let .
which is holomorphic on , hence after , for
That is the principlal part of at , and so is the principle part, after , of at infinity.
Thus is the principle part of at infinity.