title: correct definition of a differential equation for julia/DifferentialEquations
body: <p>I have two variants of a code to solve an ODE system  y'=A*y+b.  This one doesn't work (it runs but gives the wrong result, namely it returns the initial data u0 as a constant solution):</p>
  
  <pre><code>using LinearAlgebra
  using DifferentialEquations
  
  function RHSfun(du,u,p,t)
      du= p[1]*u + p[2]
      return du
  end
  
  
  A=[-1 0 0; 0 -1 0; 0 0 -1]
  
  b=[1 ; 2 ; 3 ]
  
  parm = [A,b]
  
  
   tspan=(0.0,5.0)
   u0=[0.0; 0.0 ; 0.0]
   prob=ODEProblem(RHSfun,u0,tspan,parm)
   sol=solve(prob)
  </code></pre>
  
  <p>but if I delete the argument du from RHSfun it works, which seems counter to the documentation (and I have solved ODEs defined with the du argument before). </p>
  
  <p>I am not asking for other ways of writing the problem  (i know of several working versions), but want to know why the above does not work. I have solved ODEs with the du argument before, as this is what most tutorial say to do.</p>