Signature of links

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Let L be an oriented tame link in the three sphere S 3 {S^3} . We study the Murasugi signature, σ ( L ) \sigma (L) , and the nullity, η ( L ) \eta (L) . It is shown that σ ( L ) \sigma (L) is a locally flat topological concordance invariant and that η ( L ) \eta (L) is a topological concordance invariant (no local flatness assumption here). Known results about the signature are re-proved (in some cases generalized) using branched coverings.