This N-body code Xitau computes orbital evolution of multiple stars,
accounting for *mutual* gravitational interactions. It includes a chi^2
metric computation and a simplex algorithm. It was originally designed for
the solution of \xi Tauri problem (published in Nemravova et al. 2016).

If you use this code, please cite the original references:

  M. Bro\v z, 2017, An advanced N-body model for interacting multiple stellar systems,
  Astrophys. J. Suppl. S., 230, 19, 10pp.

  J. Nemravov\'a, P. Harmanec, M. Bro\v z, D. Vokrouhlick\'y, D. Mourard,
  C.A. Hummel, C.T. Bolton et al., \xi Tauri: A unique laboratory...
  A&A, 594, A55, 47pp.

Observational data (speckle-interferometry, RV's, minima timings aka TTV, ...)
have to be in suitable format, of course (see examples in DATA.tar.gz archive).
The home page is http://sirrah.troja.mff.cuni.cz/~mira/xitau/ .

--

Warning:

Results of every numerical integration depend on discretisation of the problem.
In our case, it is controlled by dt (and dtout) parameters in param.in and param_BACK.in files.
It should be a small fraction (e.g. 1/20 to 1/100) of the SHORTEST orbital period (P_1).
The size of buffers used to store the results can be set up in simplex/simplex.inc.

As usually, one should check for convergence by decreasing dt -> ~0
or eps_BS parameter. However, if dtout is set too small, it may be rather memory
consuming (if not impossible). Luckily, the code always integrates to and outputs
all "times of interest", i.e. close to the times of all observations.

--

Coordinate convention:

   y (as DE, N)            
                           
   ^    z (v_rad)          
   |                       
   |  /                    
   | /                     
   |/                      
  -|----------> x (-RA, W) 
   |                       
                           
  internal                 

  (x, y) is plane-of-sky
  x positive towards W
  y positive towards N
  no reflections
  z radial, away from observer


                      x (DE, N) 
                                
                      ^         
                      |         
                      |         
                      |         
                      |         
  (RA, E) y <---------|-        
                     /|         
                    /           
                   /            
                                
                 z (observer)   
                                
  standard stellar-astronomy             

  (x, y) is plane-of-sky
  x positive towards N
  y positive towards E
  z radial, towards observer
  PA counterclockwise from x to y
  in an eyepiece, it may by reflected

--
                                    *****
The primary time scale for Xitau is *TDB* (i.e., as in the JPL ephemerides).
                                    *****

UTC times of observations should be converted to TDB prior to computations;
otherwise, the time scales would be inconsistent!! In particular, the UTC
is discontinuous (cf. leap seconds) and we can hardly integrate the E.O.M.
and compare (discretized) continuous w. discontinuous...