def sir_dydt ( t, y ):

#*****************************************************************************80
#
## sir_dydt() evaluates the right hand side of the SIR ODE.
#
  import numpy as np

  alpha = 0.0075
  beta  = 0.0050
  gamma = 0.0010

  S = y[0]
  I = y[1]
  R = y[2]
  P = S + I + R

  dSdt = - alpha * S * I / P            + gamma * R
  dIdt =   alpha * S * I / P - beta * I
  dRdt =                       beta * I - gamma * R

  dydt = np.array ( [ dSdt, dIdt, dRdt ] )

  return dydt