Approximating solutions of the system v′=f(t,v)

### 23.4.2 Approximating solutions of the system v′=f(t,v)

This section covers using odesolve to solve first order systems of differential equations; using it to solve a single first order differential equation was discussed last section.

The odesolve can be used to solve a system of the form

x′=f(t,x),

where x=[x1,…,xn] is a list of unknown functions and f is a function of n+1 variables with an initial condition.

odesolve can take its arguments in various ways. Letting t be the independent variable and x=[x1,…,xn] be a vector of dependent variables, t0 and x0 be the initial values, t1 the place where you want the value of x, f be the function in the differential equation, f(t,x) be a list of expressions which determines the function f (see Section 8.2.1 for the difference between a function and an expression).

- odesolve takes three or four mandatory arguments and two optional arguments:
  - mandatory, mandatory arguments given by one of the following sequences:
    - f(t,x),t=t0..t1,x,x0
    - t0..t1,(t,y)↦ f(t,y),x0
    - t0..t1,f,x0
  - Optionally, curve, the symbol.
- odesolve(mandatory ⟨,curve ⟩) returns an approximate value of x(t1) with x being the particular solution of x′(t)=f(t,x(t)) statisfying the initial condition x(t0)=x0. With an optional argument of curve, the list of all the [t,[x(t)]] values that were computed by the solver are returned.

#### Examples

Solve the system of equations x′(t)=−y(t) and y′(t)=x(t):

odesolve([-y,x],t=0..pi,[x,y],[0,1])

| ⎡ ⎣ | −1.79045146764×10−15,−1 | ⎤ ⎦

Solve the system of equations x′(t)=−y(t) and y′(t)=x(t):

odesolve(0..pi,(t,v)->[-v[1],v[0]],[0,1])

or:

f(t,v):=[-v[1],v[0]]:; odesolve(0..pi,f,[0,1])

| ⎡ ⎣ | −1.79045146764×10−15,−1 | ⎤ ⎦

odesolve(0..pi/4,f,[0,1],curve)

⎡ ⎢ ⎢ ⎢ ⎢ ⎢ ⎢ ⎢ ⎢ ⎢ ⎢ ⎢ ⎢ ⎢ ⎢ ⎢ ⎢ ⎢ ⎢ ⎣ |
0.0 |
⎡ ⎣ | 0.0,1.0 | ⎤ ⎦
0.0165441856471 |
⎡ ⎣ | −0.0165434309391,0.999863148082 | ⎤ ⎦
0.0325491321983 |
⎡ ⎣ | −0.0325433851614,0.999470323763 | ⎤ ⎦
0.0486049854945 |
⎡ ⎣ | −0.0485858499906,0.998819010222 | ⎤ ⎦
⋮ | ⋮
0.747336757246 |
⎡ ⎣ | −0.679687679865,0.733501641333 | ⎤ ⎦
0.76509544295 |
⎡ ⎣ | −0.692605846268,0.721316256377 | ⎤ ⎦
0.78286231703 |
⎡ ⎣ | −0.705311395415,0.708897619897 | ⎤ ⎦
0.785398163397 |
⎡ ⎣ | −0.707106781186,0.707106781186 | ⎤ ⎦
| ⎤ ⎥ ⎥ ⎥ ⎥ ⎥ ⎥ ⎥ ⎥ ⎥ ⎥ ⎥ ⎥ ⎥ ⎥ ⎥ ⎥ ⎥ ⎥ ⎦