def sphere_dydt ( t, y ):

#*****************************************************************************80
#
## sphere_dydt() evaluates the derivative of the sphere ODE.
#
  import numpy as np

  a = 1.6
  b = 1.0
  c = 2.0 / 3.0

  p = y[0]
  q = y[1]
  r = y[2]

  dpdt = ( 1.0 / c - 1.0 / b ) * r * q
  dqdt = ( 1.0 / a - 1.0 / c ) * p * r
  drdt = ( 1.0 / b - 1.0 / a ) * q * p

  dydt = np.array ( [ dpdt, dqdt, drdt ] )

  return dydt