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Grid moves generate isotopy of oriented knots
Statement
Assume countable choice. Let be finite planar grid diagrams, with vertical segments over horizontal segments and orientation from to in rows and to in columns, representing the same oriented knot in the standard oriented three-sphere. A cyclic permutation moves the first row or column to the last position. A row commutation exchanges adjacent rows when the closed intervals between their respective - and -mark abscissae are disjoint or one lies in the interior of the other; a column commutation is defined by the same condition on the intervals between marking heights. A stabilization chooses a marked cell and temporarily removes its marking together with the other markings in its row and column. It splits that row and column into two, places the three removed markings, and adds one new -mark and one new -mark so that every row and column again has one of each. In the four-cell subdivision of the chosen cell, exactly three cells are marked: the chosen marking type appears twice, the other type once, and any one corner may be the empty cell; the two other markings removed from the original row and column are placed outside this block in the new row and column. A destabilization is the inverse operation. Then a finite sequence of cyclic permutations, commutations, stabilizations, and destabilizations carries to .
Statement references
Facts & Assumptions
Given: Countable choice and the two finite grids representing the same oriented knot, with the full marked move conventions in the Statement.
Every grid has a finite bridge-like refinement by grid moves refines any nonempty grid to a bridge-like grid by actual marked stabilizations and legal commutations, preserving the decorated planar-isotopy class after smoothing.
Reidemeister's theorem for oriented diagrams connects regular diagrams of equivalent oriented links by finitely many planar isotopies and oriented R1/R2/R3 moves under countable choice.
Planar isotopy preserves the grid-move class of bridge rectifications connects arbitrary bridge rectifications of planar-isotopic bridge diagrams by finitely many permitted grid moves.
Relative grid realizations of Reidemeister moves connects arbitrary rectifications when bridge representatives of the endpoints of a local Reidemeister move have been supplied; it covers arbitrary exterior and every oriented sign variant.
External input: Ozsvath, Stipsicz, and Szabo, Grid Homology for Knots and Links, Appendix B.4, Definition B.4.4, says every diagram can be approximated arbitrarily closely in by bridge diagrams; the proof of Proposition B.4.3 begins by isotoping the diagram to bridge position. For a regular diagram, choose this small planar isotopy so its finite transverse crossings and their decorations persist. I use this source only to supply bridge representatives; the local arbitrary-exterior move realization remains [F4]. I inspected Definition B.4.4 and the surrounding Proposition B.4.3 and Theorem B.4.1 discussion.
Proof
A knot grid is nonempty. Apply [F1] to each , obtaining a finite grid-move sequence to a bridge-like grid . Smooth as in that supplier to a bridge diagram . Its crossing decorations and strand orientation agree with the original grid diagram, and the supplier proves planar isotopy after smoothing. Thus represent the same oriented knot. The three-mark splits and strict interval commutations in [F1] are among those specified here: the retained exterior row and column partners provide the two markings outside the stabilization block in the Statement. No unlisted shared-endpoint switch has been used.
By [F2] choose a finite sequence of planar isotopies and oriented Reidemeister moves from to . Compose successive planar isotopies, and regard each remaining Reidemeister move as an edge between its two decorated planar-isotopy classes. The sequence now traverses finitely many such classes, with the first and last represented by . If there is no Reidemeister edge, these two diagrams are planar isotopic. Otherwise every intermediate representative is a regular diagram and has a bridge representative in its decorated planar-isotopy class by [F5]; choose the perturbation small enough that its crossings and decorations persist. Choose one bridge representative for each occurrence of a class in this finite sequence, keeping at its ends, and choose a bridge rectification there. The choices are finite; no stronger choice principle is invoked.
Across a Reidemeister edge, the chosen bridge representatives need only be planar isotopic to that edge's original endpoint diagrams. This is exactly the relative-representative interface of [F4], so their chosen grid rectifications are joined by actual grid moves, with every exterior strand included. Across an interval with only planar isotopy, [F3] joins the chosen rectifications. This also treats the sequence with no Reidemeister edge. The endpoint rectifications may be taken to be precisely , because both suppliers apply to arbitrary rectifications. Concatenating the finitely many constituent finite sequences gives a sequence of allowed commutations, stabilizations and destabilizations from to .
Prepend the refinement from to and append the reversed refinement from to . Reversing a stabilization gives a destabilization and reversing a legal commutation gives a legal commutation, so all operations remain allowed. This produces the required finite sequence from to . The proof needs no cyclic permutation, but allowing those additional moves in the Statement does not change the conclusion. The local suppliers cover all marking types and strand orientations, so the sequence preserves the prescribed oriented-knot convention throughout.
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Sources
- Peter S. Ozsvath, Andras I. Stipsicz, and Zoltan Szabo, Grid Homology for Knots and Links, Appendix B.4 (standard reference, not scraped)