FUNCTION PHOTRI(A,B,C)
C.----------------------------------------------------------------------
C.
C.    PHOTOS:   PHOton radiation in decays calculation of TRIangle fie
C.
C.    Purpose:  Calculation of triangle function for phase space.
C.
C.    Input Parameters:  A, B, C (Virtual) particle masses.
C.
C.    Output Parameter:  Function value =
C.                       SQRT(LAMBDA(A**2,B**2,C**2))/(2*A)
C.
C.    Author(s):  B. van Eijk                     Created at:  15/11/89
C.                                                Last Update: 02/01/90
C.
C.----------------------------------------------------------------------
      IMPLICIT NONE
      DOUBLE PRECISION DA,DB,DC,DAPB,DAMB,DTRIAN
      REAL*8 A,B,C,PHOTRI
      DA=A
      DB=B
      DC=C
      DAPB=DA+DB
      DAMB=DA-DB
      DTRIAN=SQRT((DAMB-DC)*(DAPB+DC)*(DAMB+DC)*(DAPB-DC))
      PHOTRI=DTRIAN/(DA+DA)
      RETURN
      END