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differential equation applications 199dy (1)dt= f (1) (t, y (i) ), (9.17). .. (9.18)dy (N−1)dt= f (N−1) (t, y (i) ), (9.19)where y (i) dependence in f is allowed but not any dependence on derivativesdy (i) /dt. These equations can be expressed more compactly by use of theN-dimensional vectors (indicated here in boldface) y and f:dy(t)/dt = f(t, y),⎛y (0) ⎞ ⎛(t)f (0) ⎞(t, y)y (1) (t)f (1) (t, y)y =⎜ . , f =.. ⎟ ⎜ . ... ⎟⎝ ⎠ ⎝⎠y (N−1) (t)f (N−1) (t, y)(9.19)The utility of such compact notation is that we can study the properties of theODEs, as well as develop algorithms to solve them, by dealing with the singleequation (9.20) without having to worry about the individual components. To seehow this works, let us convert Newton’s lawd 2 xdt 2 = 1 (m F t, dx )dt ,x(9.20)to standard dynamic form. The rule is that the RHS may not contain any explicitderivatives, although individual components of y (i) may represent derivatives. Topull this off, we define the position x as the dependent variable y (0) and the velocitydx/dt as the dependent variable y (1) :y (0) (t) def= x(t), y (1) (t) def= dxdt = d(0)dt . (9.21)The second-order ODE (9.20) now becomes two simultaneous first-order ODEs:dy (0)dt= y (1) (t),dy (1)dt= 1 m F (t, y(0) ,y (1) ). (9.22)−101COPYRIGHT 2008, PRINCET O N UNIVE R S I T Y P R E S SEVALUATION COPY ONLY. NOT FOR USE IN COURSES.ALLpup_06.04 — 2008/2/15 — Page 199