Milnor invariants of welded links.

For a classical link, Milnor defined a family of isotopy invariants, called Milnor $\overline{\mu}$-invariants. In this paper, we show that Milnor $\overline{\mu}$-invariants can be extended to welded links. The proof contains an alternative and combinatorial proof for the invariance of the original $\overline{\mu}$-invariants of classical links.