real function M2_1_MINUS ( p1, p2, p3 )

c  Glen Cowan
c  12 December, 1999
c  Matrix element squared for 3-body decay of J^P = 1- particle.
c  P. Nyborg, Notes on Phase Space, p. 69, in M. Nikolic (ed.), 
c  Kinematics and Multiparticle Systems, Gordon and Breach, 1968.

       implicit             NONE

c  arguments

       real          p1 (*)
       real          p2 (*)
       real          p3 (*)

c  local variables

       integer       i

       real          m(3)
       real          m1(3), m2(3), m3(3)

c begin

       call CROSS (p1, p2, m3)                ! cernlib F117
       call CROSS (p2, p3, m1)
       call CROSS (p3, p1, m2)
       do i = 1, 3
         m(i) = m1(i) + m2(i) + m3(i)
       end do

       m2_1_minus = m(1)**2 + m(2)**2 + m(3)**2 

       return
       END