title: Find minimal key for relation scheme
body: <p>I have a database for an investment firm:</p>
  
  <ul>
  <li>B (broker)</li>
  <li>O (office of broker)</li>
  <li>I (investor)</li>
  <li>S (stock)</li>
  <li>Q (quantity of stock owned by investor)</li>
  <li>D (dividend paid by stock)</li>
  </ul>
  
  <p>Functional dependencies</p>
  
  <ul>
  <li>S ⟶ D</li>
  <li>I ⟶ B</li>
  <li>IS ⟶ Q</li>
  <li>B ⟶ O</li>
  </ul>
  
  <p>I need to find minimal key for relation scheme R=BOSQID and need to prove it.</p>
  
  <p>I have no idea how to solve this problem.
  Can you give me any idea?</p>