title: pagerank in igraph with isolated node
body: <p>I wonder how the page_rank() in R's igraph package workes when there are isolated nodes. For example,</p>
  
  <pre><code>g &lt;- graph(edges=c(1,2), n = 3, directed = F)
  page_rank(g, algo = "prpack")
  </code></pre>
  
  <p>I got (with default damping factor being 0.85.)</p>
  
  <pre><code>$vector
  [1] 0.46511628 0.46511628 0.06976744
  </code></pre>
  
  <p>Why is this result? I thought node 3 should be <code>0.15 / 3</code>.</p>
  
  <p><a href="https://i.stack.imgur.com/nSDfz.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/nSDfz.png" alt="enter image description here"></a></p>