Kervaire semi-characteristic

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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

${\displaystyle k_{F}(M)=\sum _{i=0}^{2n}\dim H^{2i}(M,F){\bmod {2}}}$

where F is a field.

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]

The difference ${\displaystyle k_{\mathbb {Q} }(M)-k_{\mathbb {Z} /2}(M)}$ is the de Rham invariant of ${\displaystyle M}$.^[2]