Kervaire semi-characteristic

In mathematics, the Kervaire semi-characteristic, introduced by Michel Kervaire (1956), is an invariant of closed manifolds M of dimension ${\displaystyle 4n+1}$ taking values in ${\displaystyle \mathbb {Z} /2\mathbb {Z} }$, given by

${\displaystyle k(M)=\sum _{i=0}^{2n}\dim H^{2i}(M,\mathbb {R} ){\bmod {2}}}$.

Michael Atiyah and Isadore Singer (1971) showed that the Kervaire semi-characteristic of a differentiable manifold is given by the index of a skew-adjoint elliptic operator.

Assuming M is oriented, the Atiyah vanishing theorem states that if M has two linearly independent vector fields, then ${\displaystyle k(M)=0}$.[1]