using Dates
using DifferentialEquations
using Plots

function vanderpol!( du, u, p, t )
#
#  The van der Pol equation:
#
#    u'' - mu * ( 1 - u^2 ) * u' + u = 0
#
  mu = 2.5
  du[1] = u[2]
  du[2] = mu * ( 1.0 - u[1] * u[1] ) * u[2] - u[1]
end

println( now( ) )
println( "DifferentialEquations.Midpoint_test():" )
println( "  Julia version" )
println( "  Test DifferentialEquations.Midpoint() on van der Pol equation." )
#
#  The initial condition, y = -0.3, y' = -0.2
#
u0 = [ -0.3; -0.2 ]
tspan = ( 0.0, 12.0 )
problem = ODEProblem( vanderpol!, u0, tspan )
sol = solve( problem, Midpoint() )
plot( sol )
#
#  Save the plot in a PNG file.
#
filename = "DifferentialEquations.Midpoint_test.png"
savefig( filename )
println( "" )
println( "  Graphics saved as '" * filename * "'" )
#
#  Terminate.
#
println( "" )
println( "DifferentialEquations.Midpoint_test():" )
println( "  Normal end of execution." )
println( now( ) )