# The co-rank of the fundamental group: the direct product, the first Betti number, and the topology of foliations

### Author: Irina Gelbukh

Mathematica Slovaca, vol. 67, no. 3, pp. 645-656, 2017.

Abstract: We study $b'_1(M)$, the co-rank of the fundamental group of a smooth closed connected manifold $M$. We calculate this value for the direct product of manifolds. We characterize the set of all possible combinations of $b'_1(M)$ and the first Betti number $b_1(M)$ by explicitly constructing manifolds with any possible combination of $b'_1(M)$ and $b_1(M)$ in any given dimension. Finally, we apply our results to the topology of Morse form foliations. In particular, we construct a manifold $M$ and a Morse form $\omega$ on it for any possible combination of $b'_1(M)$, $b_1(M)$, $m(\omega)$, and $c(\omega)$, where $m(\omega)$ is the number of minimal components and $c(\omega)$ is the maximum number of homologically independent compact leaves of $\omega$.

Keywords: co-rank, inner rank, manifold, fundamental group, direct product, Morse form foliation

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