# Signature (topology)

In the field of topology, the **signature** is an integer invariant which is defined for an oriented manifold *M* of dimension divisible by four.
This invariant of a manifold has been studied in detail, starting with Rokhlin's theorem for 4-manifolds, and Hirzebruch signature theorem.

## Definition

Given a connected and oriented manifold *M* of dimension 4*k*, the cup product gives rise to a quadratic form *Q* on the 'middle' real cohomology group

- [math]\displaystyle{ H^{2k}(M,\mathbf{R}) }[/math].

The basic identity for the cup product

- [math]\displaystyle{ \alpha^p \smile \beta^q = (-1)^{pq}(\beta^q \smile \alpha^p) }[/math]

shows that with *p* = *q* = 2*k* the product is symmetric. It takes values in

- [math]\displaystyle{ H^{4k}(M,\mathbf{R}) }[/math].

If we assume also that *M* is compact, Poincaré duality identifies this with

- [math]\displaystyle{ H^{0}(M,\mathbf{R}) }[/math]

which can be identified with [math]\displaystyle{ \mathbf{R} }[/math]. Therefore the cup product, under these hypotheses, does give rise to a symmetric bilinear form on *H*^{2k}(*M*,*R*); and therefore to a quadratic form *Q*. The form *Q* is non-degenerate due to Poincaré duality, as it pairs non-degenerately with itself.^{[1]} More generally, the signature can be defined in this way for any general compact polyhedron with *4n*-dimensional Poincaré duality.

The **signature** [math]\displaystyle{ \sigma(M) }[/math] of *M* is by definition the signature of *Q*, that is, [math]\displaystyle{ \sigma(M) = n_+ - n_- }[/math] where any diagonal matrix defining *Q* has [math]\displaystyle{ n_+ }[/math] positive entries and [math]\displaystyle{ n_- }[/math] negative entries.^{[2]} If *M* is not connected, its signature is defined to be the sum of the signatures of its connected components.

## Other dimensions

If *M* has dimension not divisible by 4, its signature is usually defined to be 0. There are alternative generalization in L-theory: the signature can be interpreted as the 4*k*-dimensional (simply connected) symmetric L-group [math]\displaystyle{ L^{4k}, }[/math] or as the 4*k*-dimensional quadratic L-group [math]\displaystyle{ L_{4k}, }[/math] and these invariants do not always vanish for other dimensions. The Kervaire invariant is a mod 2 (i.e., an element of [math]\displaystyle{ \mathbf{Z}/2 }[/math]) for framed manifolds of dimension 4*k*+2 (the quadratic L-group [math]\displaystyle{ L_{4k+2} }[/math]), while the de Rham invariant is a mod 2 invariant of manifolds of dimension 4*k*+1 (the symmetric L-group [math]\displaystyle{ L^{4k+1} }[/math]); the other dimensional L-groups vanish.

### Kervaire invariant

When [math]\displaystyle{ d=4k+2=2(2k+1) }[/math] is twice an odd integer (singly even), the same construction gives rise to an antisymmetric bilinear form. Such forms do not have a signature invariant; if they are non-degenerate, any two such forms are equivalent. However, if one takes a quadratic refinement of the form, which occurs if one has a framed manifold, then the resulting ε-quadratic forms need not be equivalent, being distinguished by the Arf invariant. The resulting invariant of a manifold is called the Kervaire invariant.

## Properties

- Compact oriented manifolds
*M*and*N*satisfy [math]\displaystyle{ \sigma(M \sqcup N) = \sigma(M) + \sigma(N) }[/math] by definition, and satisfy [math]\displaystyle{ \sigma(M\times N) = \sigma(M)\sigma(N) }[/math] by a Künneth formula.

- If
*M*is an oriented boundary, then [math]\displaystyle{ \sigma(M)=0 }[/math].

- René Thom (1954) showed that the signature of a manifold is a cobordism invariant, and in particular is given by some linear combination of its Pontryagin numbers.
^{[3]}For example, in four dimensions, it is given by [math]\displaystyle{ \frac{p_1}{3} }[/math]. Friedrich Hirzebruch (1954) found an explicit expression for this linear combination as the L genus of the manifold.

- William Browder (1962) proved that a simply connected compact polyhedron with 4
*n*-dimensional Poincaré duality is homotopy equivalent to a manifold if and only if its signature satisfies the expression of the Hirzebruch signature theorem.

- Rokhlin's theorem says that the signature of a 4-dimensional simply connected manifold with a spin structure is divisible by 16.

## See also

## References

- ↑ Hatcher, Allen (2003) (in en).
*Algebraic topology*(Repr. ed.). Cambridge: Cambridge Univ. Pr.. p. 250. ISBN 978-0521795401. https://www.math.cornell.edu/~hatcher/AT/AT.pdf. Retrieved 8 January 2017. - ↑ Milnor, John; Stasheff, James (1962) (in en).
*Characteristic classes*. Annals of Mathematics Studies 246. p. 224. ISBN 978-0691081229. - ↑ Thom, René. "Quelques proprietes globales des varietes differentiables" (in fr). Comm. Math. Helvetici 28 (1954), S. 17–86. https://www.maths.ed.ac.uk/~v1ranick/papers/thomcob.pdf. Retrieved 26 October 2019.

Original source: https://en.wikipedia.org/wiki/Signature (topology).
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