FOF(1)                         FOF                         FOF(1)



NAME
     fof  -  Find  groups  in  N-body   simulations   using   the
     friends-of-friends method.

SYNOPSIS
     fof -e <Linking Length> [-m <nMinMembers>] [-dgs] [-v]
          [-o <Output Name>] [-p <xyzPeriod>]
          [-px <xPeriod>] [-py <yPeriod>] [-pz <zPeriod>]

     Reads particles from stdin in the tipsy binary format.


OPTIONS
     -e <Linking Length>: No Default fEpsilon value!
     This argument specifies  the  linking  length  used  by  the
     friends-of-friends method.

     [-m <nMinMembers>]: Default nMinMembers = 8
     This argument allows the user to specify  what  the  minimum
     number  of  members  (particles)  per  group  should be. All
     groups with less than this value will be dicarded.

     [-dgs]: Default: all particles considered (-dgs)
     This set of flags allows the user to  specify  the  particle
     types  to  be considered for grouping. If the user specifies
     -d, then only the dark matter particles are considered. With
     -dg,  both  dark  and  star particles are considered but any
     star particles are  ignored.  The  default  is,  effectively
     -dgs.

     [-v]: Default is no output
     This flag allows the user to enable  diagnostic  output  (on
     stdout).  This  includes the number of groups   found by the
     friends-of-friends method, the number of groups meeting  the
     minimum  members  criterion  and also the CPU time needed to
     find the groups.

     [-o <Output Name>]: Default Output Name = "fof".
     This allows the user to specify an alternate  name  for  the
     output  files. For example, if the user specifies -o sim5 on
     the command line, then fof will by default produce the  file
     sim5.grp.

     [-px <xPeriod>]: Default non-periodic in x dimension
     Specifies that fof take  into  account  periodicity  in  the
     x-dimension given by the period <xPeriod>.

     [-py <yPeriod>] [-pz <zPeriod>]: As above.

     [-p <xyzPeriod>]: Default non-periodic in x,y and  z  dimen-
     sions
     Specifies that fof take  into  account  periodicity  in  all
     three dimension given by the period <xyzPeriod>. This can be
     used in place of the above as a shorthand.


DESCRIPTION
     The fof program finds groups in N-body simulations using the
     friends-of-friends method. It rejects any groups found which
     have  less  than  nMinMembers   number   of   particles.   A
     friends-of-friends  group  is one in which every particle in
     the group has a "friend" particle  within  a  distance  less
     than  or equal to some specified "linking length". The link-
     ing length is set with the -e <Linking Length> argument.


OUTPUT FILE
     fof.grp : This ASCII file is in TIPSY ARRAY format and  con-
     tains  the  group number to which each particle in the input
     file belongs. Group number zero means this particle was  not
     grouped.  This  file  can  be  read in by tipsy or any other
     analysis tool able to read this format.


EXAMPLES
     > fof -e 1.0e-4 -p 1 -o dark < dark.bin

     This example groups all particles in the file dark.bin.  The
     simulation  is  periodic  with  period length of 1.0 in each
     dimension. The minimum group members  is  taken  to  be  the
     default value of 8 and the name of the output file is speci-
     fied to be dark.grp.

     > fof -e 1.0 -v -d -m 4 < gasrun.bin

     This example groups only the dark matter  particles  in  the
     input  file  gasrun.bin.  The simulation state is treated as
     non-periodic, since there are no periods specified.  Verbose
     (-v) ouput is requested. The number of minimum group members
     has been reduced to 4 and the name of the output  file  will
     be fof.grp.


WARNINGS
     If the linking length is set  unreasonably  large  or  if  a
     large  number of particles are found within a ball of radius
     the linking length, then the fof program will take VERY long
     to  complete.  The reason for this is that in such cases the
     algorithm used here degenerates to  O(N^2).   For  typically
     desired  linking lengths the fof program completes in a rea-
     sonable amount of time.


BUGS
     Please report any!


SEE ALSO
     tipsy(1), smooth(1), denmax(1)


Stadel Release 1.0  Last change: July 1, 1994