Novikov additivity
= Novikov additivity
{c}
When compact oriented four-manifolds are glued along an entire closed boundary three-manifold, the <signature> of the result is the sum of their <intersection form> signatures. It applies to $X_0\cup_{\partial X_0}(-X_1)$ with the difference of the two signatures. Gluing along only part of a boundary can require a correction; that is a different situation.