= Intersection form
{title2=$Q_M$}
For an oriented compact manifold of dimension $4k$, the intersection form is the bilinear pairing on middle-dimensional homology obtained by representing classes by transverse cycles and counting their signed intersection points. On an oriented four-manifold it is equivalently the cup-product pairing on $H^2$ evaluated on the fundamental class.
The <intersection form of a 4-manifold> is the four-dimensional case of this middle-dimensional intersection pairing.
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