### 27.1 Defining the ODE System

The routines solve the general n-dimensional first-order system,

```dy_i(t)/dt = f_i(t, y_1(t), ..., y_n(t))
```

for i = 1, \dots, n. The stepping functions rely on the vector of derivatives f_i and the Jacobian matrix, J_{ij} = df_i(t,y(t)) / dy_j. A system of equations is defined using the `gsl_odeiv2_system` datatype.

Data Type: gsl_odeiv2_system

This data type defines a general ODE system with arbitrary parameters.

`int (* function) (double t, const double y[], double dydt[], void * params)`

This function should store the vector elements f_i(t,y,params) in the array dydt, for arguments (t,y) and parameters params.

The function should return `GSL_SUCCESS` if the calculation was completed successfully. Any other return value indicates an error. A special return value `GSL_EBADFUNC` causes `gsl_odeiv2` routines to immediately stop and return. If `function` is modified (for example contents of params), the user must call an appropriate reset function (`gsl_odeiv2_driver_reset`, `gsl_odeiv2_evolve_reset` or `gsl_odeiv2_step_reset`) before continuing. Use return values distinct from standard GSL error codes to distinguish your function as the source of the error.

`int (* jacobian) (double t, const double y[], double * dfdy, double dfdt[], void * params);`

This function should store the vector of derivative elements in the array dfdt and the Jacobian matrix J_{ij} in the array dfdy, regarded as a row-ordered matrix `J(i,j) = dfdy[i * dimension + j]` where `dimension` is the dimension of the system.

Not all of the stepper algorithms of `gsl_odeiv2` make use of the Jacobian matrix, so it may not be necessary to provide this function (the `jacobian` element of the struct can be replaced by a null pointer for those algorithms).

The function should return `GSL_SUCCESS` if the calculation was completed successfully. Any other return value indicates an error. A special return value `GSL_EBADFUNC` causes `gsl_odeiv2` routines to immediately stop and return. If `jacobian` is modified (for example contents of params), the user must call an appropriate reset function (`gsl_odeiv2_driver_reset`, `gsl_odeiv2_evolve_reset` or `gsl_odeiv2_step_reset`) before continuing. Use return values distinct from standard GSL error codes to distinguish your function as the source of the error.

`size_t dimension;`

This is the dimension of the system of equations.

`void * params`

This is a pointer to the arbitrary parameters of the system.