SSL Minutes Thursday, March 25 2004 Notetaker today: Sam Snacks today: Emilie Notetaker for next time: Hal Snacks for next time: Sam ANNOUNCEMENT: There will be a party after Monday's Math Club meeting on account of the high praise received regarding the NSF meeting. It will be after Jesse Beder's talk on Braid Groups and all are invited. Paul asks a question about edge variables and lifting recurrences into higher dimensional contexts. Jim: For example, in the following recurrence we want to find suitable B,C,D,E so that Ba(n)a(n-k) = Ca(n-i)a(n-k+i) + Da(n-j) + Ea(n-k+j) This will help us understand combinatorial data of the recurrence which is hidden (by the fact that certain "edges" have weight 1). Sam: I found the algebraic identity which forms the last piece of the puzzle in the formulas for P_n and Q_n. Jim: Now that we have a finished proof, we want to work on writing this up. The next order of business may involve looking at 1) connection with the continued fraction algorithm, 2) combinatorial proof, and 3) q-analogs. In fact, during the rest of the term, we should shift our focus towards documenting the results we have found. People should work together, read each other's write-ups, and unify their results. Paul asks about what combinatorial data we can glean once we have faithful polynomials. Jim: It turns out there are situations where not all combinatorial properties can be deduced from the faithful polynomials straightforwardly. Example given is a straight snake graph, where all the vertical edges have weights, and the horizontal ones are "weightless," (i.e. weight 1). By putting in extra variables, we can get more clues about the combinatorial picture, vis-a-vis the revealing of these hidden edges. My previous e-mail fully delineates this idea for a specific sequence. There are some LaTex questions floated about. Carl asks about greatest integer symbol (which is \lfloor or \rfloor), and we also have \lceil and \rceil). Jim points out the utility of \eqnarray for lining up a bunch of equations. Jon answers Hal's questions about limits of summation in a fractional expression: trick is to use \sum\limits forcing them to be on top. Martin discusses some of his computer results regarding the Markoff mod p graphs. He has written programs to calculate the number of triples which solve the Markoff relation and the order of the automorphism group of the graph. For the Markoff relation, |Aut(G)| = 24 regardless of p (once p is large enough). He has also studied the Markoff brothers, Chico (1,1,2,4) and Groucho (1,2,3,6). He has found that whenever the number of triples solving either the Chico or Groucho relation is the same as the number of triples which solve the original Markoff relation, then the order of the automorphism group is 24. Otherwise, its order is 8. Not sure what this result entails exactly. One thing to notice is that in the original Markoff relation (1,1,1,3), we can permute the variables x,y,z. So we can get 3! = 6 symmetries of the graph this way. Also, we can change the sign or any *two* variables (either (x,y), (x,z), (y,z)) or none at all. This gives us 4 more symmetries. So we have 6*4 = 24, the order of the automorphism group. Jim: Initially we were interested in studying possible expander properties of these graphs. Here we might want to consider the spectrum, particularly the "spectral gap": the difference between the two greatest eigenvalues. Martin will see what he and Magma can do in terms of programming, and will do some research regarding an explicit algorithm for the expander property. Paul and Emilie will be working on writing up some of their results regarding the generalized recurrences exhibiting Laurentness. And Paul will also work on writing up some results regarding cube snakes. (BREAK) We reconvene in B107 to work on our group projects. Hal, Carl, Jon, and Sam discuss aspects of writing up Newton's method and the perspective the authors should adopt. Jim, Emilie, and Paul work together on the recurrences.