# Docs DifferentialEquations.jl SecondOrderODEProblem

What does the symbol f (as opposed to f1 or f2) in

SecondOrderODEProblem{isinplace}(f,du0,u0,tspan,p=NullParameters();kwargs…)

on the page Dynamical, Hamiltonian, and 2nd Order ODE Solvers · DifferentialEquations.jl refer to? That is, how is the second order ODE defined?

I saw the example at 2nd Order ODEs in Julia using DifferentialEquations - Stack Overflow

Possibly, however, it might be valuable to make the documentation more explicit.

Thanks.

The answer to my question is given at Dynamical, Hamiltonian and 2nd Order ODE Problems · DifferentialEquations.jl , i.e., the problems page (instead of the solvers page).

Thx for looking along. Domenico.