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#! /usr/bin/env python
import openturns as ot
import math as m
ot.TESTPREAMBLE()
f = ot.SymbolicFunction(["t", "y0", "y1"], ["t - y0", "y1 + t^2"])
nt = 100
timeGrid = [(i**2.0) / (nt - 1.0) ** 2.0 for i in range(nt)]
phi = ot.ParametricFunction(f, [0], [0.0])
solver = ot.RungeKutta(phi)
print("ODE solver=", solver)
initialState = [1.0, -1.0]
print("time grid=", ot.Point(timeGrid))
result = solver.solve(initialState, timeGrid)
print("result=", result)
print("last value=", result[nt - 1])
t = timeGrid[nt - 1]
ref = ot.Point(2)
ref[0] = -1.0 + t + 2.0 * m.exp(-t)
ref[1] = -2.0 + -2.0 * t - t * t + m.exp(t)
print("ref. value=", ref)
grid = ot.RegularGrid(0.0, 0.01, nt)
result = solver.solve(initialState, grid)
print("result=", result)
print("last value=", result[nt - 1])
t = grid.getValue(nt - 1)
ref[0] = -1.0 + t + 2.0 * m.exp(-t)
ref[1] = -2.0 + -2.0 * t - t * t + m.exp(t)
print("ref. value=", ref)
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