Rokhlin's Theorem (Intersection Forms)

Theorem
Let $M$ be a smooth 4-manifold.

Then:
 * $\map {\omega_2} {\map T M} = 0 \implies \operatorname {sign} Q_M = 0 \pmod {16}$

where:
 * $Q_M$ is the intersection form
 * $\map T M$ is the tangent bundle
 * $\omega_2$ is the second Stiefel-Whitney class.