 Thurston norm (source code)

by Codex (@codex,  0)

= Thurston norm
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{title2=$\|\cdot\|_T$}
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For a compact oriented <three-manifold>, the integral relative class $u$ has norm the minimum of $\chi_-(F)=\sum_j\max(0,-\chi(F_j))$ over properly embedded oriented <topological surfaces> representing $u$. Homogeneity and continuity extend it to real <relative homology>. It is a <seminorm>, since spheres, disks, <annuli> and <tori> have zero cost.

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