VasCalc Documentation - User's Guide
This page documents the ChordsMod4T component of the VasCalc project. This page is also under construction.
Warning (08/19/06): The documentation below may be slightly out of date, as we are in the process of changing the
ASeries notation. This should be resolved within a few days - please bear with us.
This package provides a Mathematica interface to work with the space generated by chord diagrams on a fixed skeleton of lines and circles, modulo the 4-T relation. See Dror Bar-Natan's Survey of Finite Type Invariants for further details.
The software requirements of this package are Mathematica 4.1 and Java 1.4 (or greater, in both cases). However, note that default installations of Mathematica 4.1 and up include a Java runtime binary, and require no further configuration for this package to work.
While there are no specific hardware requirements, the computations involved are rather resource-intensive. Users of low-end machines will see a smaller range of parameters for which the computation is practicable. We'll have an expected performance profile once work on this package is done.
You must also ensure that there is a directory named "CM4TData" under the installation directory, creating it if neccessary. This is for save/load functionality to work properly.
In a Mathematica session, load the definitions by typing:
where "path_to_install_folder" is either the "trunk" folder of the repository, or the folder in which the above archive, if downloaded, was unpacked.
You must also issue the following command, to inform Mathematica of the location of the required Java objects:
These steps must be followed at the start of each Mathematica session you wish to use this package.
Representing Chord Diagrams
Our convention for representing a chord diagram in Mathematica is through a
CD object containing
Circle expressions as per the skeleton. To construct such a representation,
- Number the chords on the diagram.
- For each line on the skeleton, put within the brackets of its
Lineexpression the numbers of the chords that have their endpoints on that line, starting from the bottom and working up. For each circle, do the same, but one can choose an arbitrary starting point and count around. If there are no chords on a given line or circle, leave the brackets empty, as in
Naturally, there are many representations for a given diagram.
This package also allows for formal linear combinations of
Computing chord diagram spaces modulo 4T and FI relations
CDSpace[l,m,n] represents the rational vector space generated by chord diagrams with
n chords on a skeleton with
l lines and
m circles, modulo the 4-T relation. The first time a space is encountered in a session, a check is made to see if there is data for the required space on disc (under the path_to_install_folder/CM4TData directory). If no such entry exists, the space is computed, and by default the results are saved on disc for future use - this computation may be substantial, depending on the values of the parameters. One can prevent the saving of data by using the command
CDSpace[l,m,n, WriteToDisc -> False] .
One can obtain the dimension of a chord diagram space, as follows:
One can also obtain a basis for a chord diagram space (modulo 4T), expressed as a list of chord diagrams:
In addition, one can work with chord diagram spaces modulo the 4T and framing independence (FI) relations, using the command
CDrSpace[l,m,n,opts] - the "r" stands for "further reduced". The above commands work in a similar fashion for these spaces.
Because operations on chord diagrams (such as multiplication and reduction) can often be resource-intensive, it is desirable to work with representations in Java as much as possible. The
ASeries object is meant to facilitate this. To create one, use the command
ASeries[Permutation, m, CDcombo], where
Permutationis a list of integers from 1 to
l(the number of strands) which encode the permutation of those strands that the
ASeriesis intended to represent (Note -
Id[n]can be used as a shortcut for the identity permutation on
nstrands). For example, if one wishes a combination of chord diagrams to represent the braiding of the first two strands in a skeleton of five strands, one would use the permutation
mis the number of circles, and
CDcombois a linear combination of chord diagrams (represented by
CD's, and possibly a constant term), all of which are on a fixed skeleton of
mcircles, and may have a different number of chords.
It is important to note that an
ASeries carries with it the notion of an order, defined implicitly by the maximum number of chords to appear in a diagram in
CDcombo; the result of any and all computations are only given up to the lowest order appearing therein. To force a maximum order, use the command
ASeries[Permutation, m, CDCombo, MaxDegree], where
MaxDegree is a non-negative integer.
There are several operations one can perform on an
ASeries object. Addition and multiplication is achieved using the
. operators respectively - however only diagrams representing the same permutation can be added. Multiplication naturally composes the permutations. Also,
ASeries objects can be inverted (up to the highest order), provided they are of the form
(1 + higher order terms) .
Additionally, the following unary operations are available:
AddStrand[n, ASeries]adds an empty strand to all the diagrams in the given
DoubleStrand[n, ASeries]doubles the
n'th strand in all the diagrams in the given
ReverseStrand[n, ASeries]reverses the
n'th strand in all the diagrams in the given
ConnectStrand[i, j, ASeries]is the linear extension of the operation that for each diagram, appends the
j'th strand to the end of the
ASeriesequivalent to the first modulo the 4T relation. This operation may take some time if the relevant chord diagram spaces need to be computed.
ASeriesequivalent to the first modulo the 4T relation, and the FI relations. As before, this operation may take some time if the relevant chord diagram spaces need to be computed.
All these operations will be put to use in the next section.
Finally, to convert an
ASeries back to a human-readable form, use the command:
CD[ AS ], where
AS is the object to be converted.
Special Form of ASeries
When calculating the invariant of a knot or a braid, we multiply the invariant of
the different "events" in the knot's presentation. In the case where this event
is a crossing, one must keep in mind that two strands have been permuted. For
this reason one must apply
PermuteStrand in order to get the
correct answer. This causes the resulting expressions to be rather inelegant.
For this reason we have devised a new form of
ASeries that keeps
track of the permutation of the strands so one does not need to manually permute
them when multiplying.
This format only applies when there are no circles in our ASeries, as in the
case of knots and braids. One can then define an ASeries using the format:
ASeries[Permutation_list, CV_list]. Here,
Permutation is a list
of integers from to </math>n</math>, where is
the number of lines in the diagram.
CV is a list containing Chord vectors
of increasing degree.
Checking the pentagon and hexagon relations
For paranthesized braids, the pentagon and hexagon equations are given by
|The Pentagon and the Hexagons for Parenthesized Braids|
Algebraically, these relations can be written as
where is the algebra of chord diagrams on strands modulo the 4-T relations.
Let us check that a proposed pair satisfy these equations, up to the third order. After running through the initialization above, we introduce our candidates as
First, we'll check the pentagon equation. Writing the difference of the two sides for ,
Now we reduce the result modulo the 4T relations, and convert back to
Thus our candidate solves the pentagon equation.
Let's also test one of the hexagon equations:
Reducing the result, we see that our choices of and yield a solution to the first hexagon equation.
The second hexagon equation can be verified in a similar fashion, and the result is the same.
To come: definition of Z[B[n]], demonstration of Reidemeister moves. For now, reidemeister.nb.
(Unframed) Knot Invariants
To compute for a knot, we need to extend the definition of to include "caps" and "cups". In addition, framing independence forces us to introduce the FI relation - hence all the formulae in this section take place in the "further reduced" space of chord diagrams. We proceed using the correction term in . To begin, let us choose a pair which solve the pentagon and hexagon equations to fourth degree:
The definition of involves going up the third strand, down the second, and up the first strand of :
The invariant of the unknot is given by closing to a circle (the first line below), and taking the inverse:
We are currently working to improve the performance of this program. For larger computations, it would likely help to increase the maximum heap size available to Java. To do this, type the following into Mathematica before loading the CDinterface.m definitions (step 1 above):
This allows Java to use up to 128 megabytes of memory; you may change the number "128" in the parameter "-Xmx128M" to suit your needs. Note that the first line needs to be entered exactly as shown - the funny apostrophe is located next to the "1" key on many keyboards.
-  D. Bar-Natan, Non-Associative Tangles. Geometric Topology (proceedings of the Georgia International Topology Conference, W. H. Kazez ed.), 139-183, Amer. Math. Soc. and International Press, Providence, 1997.