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Acta Mathematica Vietnamica

Balloons and Hoops and their Universal Finite-Type Invariant, BF Theory, and an Ultimate Alexander Invariant

icon-email Dror Bar-Natan

Abstract

Balloons are two-dimensional spheres. Hoops are one dimensional loops. Knotted Balloons and Hoops (KBH) in 4-space behave much like the first and second homotopy groups of a topological space --- hoops can be composed as in $\pi_1$, balloons as in $\pi_2$, and hoops ``act'' on balloons as $\pi_1$ acts on $\pi_2$. We observe that ordinary knots and tangles in 3-space map into KBH in 4-space and become amalgams of both balloons and hoops. We give an ansatz for a tree and wheel (that is, free-Lie and cyclic word) -valued invariant $\zeta$ of (ribbon) KBHs in terms of the said compositions and action and we explain its relationship with finite type invariants. We speculate that $\zeta$ is a complete evaluation of the BF topological quantum field theory in 4D. We show that a certain ``reduction and repackaging'' of $\zeta$ is an ``ultimate Alexander invariant'' that contains the Alexander polynomial (multivariable, if you wish), has extremely good composition properties, is evaluated in a topologically meaningful way, and is least-wasteful in a computational sense. If you believe in categorification, that should be a wonderful playground.