Apples-with-Apples Retreat
LSU, Baton Rouge, Louisiana, April 2004

Summary

An Apples with Apples retreat was held at LSU from April 26 to 28, 2004, in conjunction with a Cactus retreat. We thank all our LSU colleagues, especially Manuel Tiglio, for their hospitality.
The primary goal of the meeting was to make progress toward writing a report on the first round of Mexico tests. Similar sessions had previously been held at AEI among the local participants but in Baton Rouge we benefited from a wider representation of participating groups. Ian Hawke led a tour of the CVS output for the first round of Mexico tests. The results were discussed. Then small work groups were formed which updated the draft paper on the first round of tests to include analysis, comparisons and conclusions for the robust stability tests and gauge wave tests. Another work group was formed to clean up the descriptions of the various BSSN systems, and the large amount of information on adjusted ADM and BSSN systems. The output of the 'robust' and 'gauge wave' working groups will serve as templates for adding treatments of the linear wave and Gowdy wave tests to complete the paper. A mailing list mex1results@aei.mpg.de has been created to facilitate the communication about the paper. Joining the mailing list will be understood as a commitment to authorship on the Mexico I results paper, and active participating in writing up the paper. For joining one of the working groups, please send email to the mex1results mailing list.
Decision on a schedule for completing the paper will be shortly made after all interested participating groups have been given a final chance to complete their test data in the CVS.
In the Mexico spirit, there were few formal presentations. The major topics of discussion outside of the working groups were as follows:

• Designing the next round of boundary tests. The working paper on the next round of boundary tests has been updated in the CVS under /ApplesWithApples/Papers/Boundaries1. The first round of Mexico tests on :T3 have been extended to a first round of boundary tests by open one face of the 3-torus to form a manifold with two T2 boundaries. This isolates problems due to smooth boundaries by avoiding the sharp edges and corners posed by cubic boundaries. In the robust stability boundary test the constraints are not satisfied and all variables requiring boundary data are given random data. In the linear, gauge and Gowdy wave boundary tests, the spacetime metric is known so that the correct boundary data for all variables requiring boundary data can be explicitly prescribed. Thus these tests can be run independently of a constraint preserving boundary condition. Addition of a Rindler wedge boundary test is under consideration. The linear wave test may be dropped if the Stage 1 results show that nothing essential is learned.
  The plan is to carry out preliminary exploration of a number of these tests to decide whether the suggested parameters and output data are appropriate. The general agreement is that the tests should be run with a Runge Kutta update scheme rather than ICN. Examples of boundary test runs were presented by Bela Szilagyi.
• The new organization of the CVS was explained by Sascha Husa.
• The redesign of the Apples web page was presented by Maria Babiuc and future plans were discussed.
• Tentative plans for future meetings were made. They include a November 2004 meeting in Potsdam and a Spring 2005 meeting in Cordoba.