GNU Scientific Library – Reference Manual: Defining the ODE System

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### 26.1 Defining the ODE System

The routines solve the general n-dimensional first-order system, 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. The user must call an appropriate reset function (e.g. `gsl_odeiv2_driver_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. The user must call an appropriate reset function (e.g. `gsl_odeiv2_driver_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.

Next: Stepping Functions, Up: Ordinary Differential Equations [Index]