Numerical Integration of Ordinary Differential Equations Reading: NDAC Secs. 2.8 and 6.1 Lecture NI: Nonlinear Physics, Physics 150/250 (Spring 2010); Jim Crutchfield Monday, April 19, 2010 1 Numerical Integration ... Dynamical System:{X , T} State Space: X State: x ∈X x X Dynamic: T : X→X x! x X X Lecture NI: Nonlinear Physics, Physics 150/250 (Spring 2010); Jim Crutchfield Monday, April 19, 2010 2 Numerical Integration ... Dynamical System ... For example, continuous time ... d !x˙ F! !x Ordinary differential equation: = ( ) (˙ = dt ) n State: !x (t) ∈ R !x =(x1,x2,...,xn) Initial condition: !x (0) ! n n Dynamic: F : R → R F! =(f1,f2,...,fn) Dimension: n Lecture NI: Nonlinear Physics, Physics 150/250 (Spring 2010); Jim Crutchfield Monday, April 19, 2010 3 Numerical Integration ... X = R2 x = (x1, x2) Geometric view of an ODE: F = (f1(x), f2(x)) dx = F (x) !x ! !x tF! !x dt = +∆ ( ) dx ∆x x x = − ∆x2 = f2(!x ) dt ≈ ∆t ∆t !x ∆x1 = f1(!x ) Each state !x has a vector attached F (x) that says to what next state to go: x = x + ∆ t F ( x ) . · Lecture NI: Nonlinear Physics, Physics 150/250 (Spring 2010); Jim Crutchfield Monday, April 19, 2010 4 Numerical Integration ... Vector field for an ODE (aka Phase Portrait) X = R2 A set of rules: Each state has a vector attached That says to what next state to go Lecture NI: Nonlinear Physics, Physics 150/250 (Spring 2010); Jim Crutchfield Monday, April 19, 2010 5 Numerical Integration ... Time-T Flow: T T ˙ x(T ) = φT (x(0)) = dt x = dt F (x(t)) 0 0 The solution of the ODE, starting from some IC Simply follow the arrows !x (0) !x (T ) φT Point: ODE is only instantaneous, flow gives state for any time t. Lecture NI: Nonlinear Physics, Physics 150/250 (Spring 2010); Jim Crutchfield Monday, April 19, 2010 6 Numerical Integration ... Euler Method in 1D: x˙ = f(x) dx ∆x x x = n+1 − n dt ≈ ∆t ∆t x(t + ∆t) x = x + f(x )∆t 0 ≈ 1 0 0 xn+1 = xn + f(xn)∆t A discrete-time map! Lecture NI: Nonlinear Physics, Physics 150/250 (Spring 2010); Jim Crutchfield Monday, April 19, 2010 7 Numerical Integration ... Improved Euler Method in 1D: x˙ = f(x) A trial (Euler) step: xn = xn + f(xn)∆t The resulting better estimate (averaged at t n and t n +1 ): 1 xn+1 = xn + 2 [f(xn) + f(xn)] ∆t Lecture NI: Nonlinear Physics, Physics 150/250 (Spring 2010); Jim Crutchfield Monday, April 19, 2010 8 Numerical Integration ... Fourth-order Runge-Kutta Method in 1D: Intermediate estimates: k1 = f(xn)∆t 1 k2 = f(xn + 2 k1)∆t 1 k3 = f(xn + 2 k2)∆t k4 = f(xn + k3)∆t Final estimate: 1 xn+1 = xn + 6 [k1 + 2k2 + 2k3 + k4] Good trade-off between accuracy and time-step size. Lecture NI: Nonlinear Physics, Physics 150/250 (Spring 2010); Jim Crutchfield Monday, April 19, 2010 9 Numerical Integration ... Runge-Kutta in nD: n n n x˙ = f(x), x R f : R R ∈ → Intermediate estimates: k1 = f(xn)∆t 1 k2 = f(xn + 2 k1)∆t 1 k3 = f(xn + 2 k2)∆t k4 = f(xn + k3)∆t Final estimate: 1 xn+1 = xn + 6 k1 + 2k2 + 2k3 + k4 Lecture NI: Nonlinear Physics, Physics 150/250 (Spring 2010); Jim Crutchfield Monday, April 19, 2010 10.