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DEM50

       SUBROUTINE  DEM50 (NIN,NOUT)
 C$    (DEM50 - Flow Vectors)
 C$    Demonstrate the graphing of a vector field with PLTFV.  The
 C$    illustrated  field  arises   from  Duffing's   differential
 C$    equations which represent a damped harmonic oscillator with
 C$    a non-linear perturbation.  These equations are
 C$
 C$    dx/dt = y
 C$
 C$    dy/dt = -a*x - b*y - c*x**2
 C$
 C$    where a, b, and  c are suitable  constants, usually in  the
 C$    range 0.0..1.0.  With (x,y) representing the coordinate  of
 C$    a point on a rectangular grid, (dx/dt,dy/dt) represents the
 C$    vector field at that point.
 C$    (10-APR-82)