title: Solving a matrix differential equation with Mathematica
body: <p>I need to solve this equation in Mathematica:</p>
  
  <blockquote>
    <p>d/dx <strong>v</strong>(x) = <strong>A</strong> . <strong>v</strong>(x)</p>
  </blockquote>
  
  <p>here <strong>v</strong> is the <strong>column vector</strong> <code>{v1(x),v2(x),v3(x),v4(x)}</code> and 
  <strong>A</strong> is a <strong>4 x 4 matrix</strong>. 
  I want to solve for the functions <code>v1, v2, v3, v4</code> with any initial conditions. 
  The range of x is from 0 to 1000. </p>
  
  <p>How do I write Mathematica code for this type of differential equation using <code>NDSolve</code>?</p>