Link concordance
# Link concordance

In mathematics, two links $L_{0}\subset S^{n}$ and $L_{1}\subset S^{n}$ are concordant if there exists an embedding $f:L_{0}\times [0,1]\to S^{n}\times [0,1]$ such that $f(L_{0}\times \{0\})=L_{0}\times \{0\}$ and $f(L_{0}\times \{1\})=L_{1}\times \{1\}$.

By its nature, link concordance is an equivalence relation. It is weaker than isotopy, and stronger than homotopy: isotopy implies concordance implies homotopy. A link is a slice link if it is concordant to the unlink.

## Concordance invariants

A function of a link that is invariant under concordance is called a concordance invariant.

The linking number of any two components of a link is one of the most elementary concordance invariants. The signature of a knot is also a concordance invariant. A subtler concordance invariant are the Milnor invariants, and in fact all rational finite type concordance invariants are Milnor invariants and their products, though non-finite type concordance invariants exist.

## Higher dimensions

One can analogously define concordance for any two submanifolds $M_{0},M_{1}\subset N$. In this case one considers two submanifolds concordant if there is a cobordism between them in $N\times [0,1],$ i.e., if there is a manifold with boundary $W\subset N\times [0,1]$ whose boundary consists of $M_{0}\times \{0\}$ and $M_{1}\times \{1\}.$

This higher-dimensional concordance is a relative form of cobordism – it requires two submanifolds to be not just abstractly cobordant, but "cobordant in N".

