Dror Bar-Natan: Papers: Categorification:
Explaining the "Categorification.m" Program
The purpose of this document, written on September 17 2008 along with Jana Archibald, Zsuzsanna Dancso, Iva Halacheva, Louis Leung and Krzysztof Putyra is play around a bit with Dror's old "categorification.m" mathematica program, which computes Khovanov homology. Hopefully, Krzysztof will use this knowledge to produce a program to compute his variant of "odd" Khovanov homology. The original program is available at http://www.math.toronto.edu/~drorbn/papers/Categorification/Categorification.m. It is reproduced here in (mostly) red, with minor modifications to make it compliant with KnotTheory` conventions.
This Mathematica notebook is available at http://www.math.toronto.edu/~drorbn/papers/Categorification/CategorificationProgramExplained.nb.
First, let us load KnotTheory` :
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PD conventions are explained at http://katlas.org/wiki/Planar_Diagrams.
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First, for degree reasons, we need to count the positive and negative crossings in pd :
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Let' s implement the smoothing rules : X[i, j, k, l] -> either p[i, j] p[k, l] or p[i, l] p[j, k] . First. "p" must be "commutative" or ""Orderless":
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We now go into the definition of "S", which stands for "Smoothing" : it gets an input which is a PD and either a list {0, 0, 1, 0} of zeros and ones or a string "0010".
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Constructing the vertices of the Khovanov cube
First, a simple variant:
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So our smoothing has 3 cycles!
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"p[i,j][m]" means "a path from i to j, named m; by convention, a path is named after the minimal edge inside it.
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Yet, we want S to also understand "*" s!
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Define the degree of an arbitrary combination of v_ + and v_ - ' s :
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V[L, a] is the vector space sitting at a for L, V[L, a, deg] will be the degree deg part of V[L, a]:
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Defining the edge morphisms :
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The "column" direct sums :
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The differential
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