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arXiv · 2601.12846

A diagrammatic approach to the three-page index

Hyungkee Yoo
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Abstract

The three-page index $α_3(L)$ is an invariant that measures the complexity of representing a link $L$ in a three-page book. It is known that $α_3(L)$ admits a linear upper bound in terms of the crossing number, with equality realized by the Hopf link. In this paper, we investigate the equality case of this bound from a diagrammatic viewpoint. Starting from a reduced link diagram, we construct three-page presentations via binding circles arising as boundaries of suitable contractible subcomplexes of the induced cell decomposition of the $2$-sphere. This approach allows a refined control of the number of arcs in the resulting three-page presentation. As a consequence, we prove that for any non-split, nontrivial link $L$ other than the Hopf link, \[ α_3(L)\le 3c(L)-1, \] and hence characterize completely the links for which $α_3(L)=3c(L)$.

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Hyungkee Yoo. 2026-01-19. A diagrammatic approach to the three-page index. https://arxiv.org/abs/2601.12846

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