title: How to solve this multivariate recurrence with Mathematica?
body: <p>I would like to solve this recurrence relation:<br />
  $a_{m,n}=a_{m-1,n}+a_{m,n-1}$  with $a_{0,0}=0, a_{m,0}=1, a_{0,n}=1$<br />
  Related to the Tartaglia triangle,</p>
  <p>the solution should be just the combinations...<br />
  $a{m,n}=Binomial(m+n,n)$</p>
  <p>But when I try to solve it with Mathematica</p>
  <pre><code>RSolve[{a[m, n] == a[-1 + m, n] + a[m, -1 + n], a[0, 0] == 0, 
    a[m, 0] == 1, a[0, n] == 1}, a[m, n], {m, n}]  
  </code></pre>
  <p>It just outputs the same input unevaluated.</p>
  <p>What am I doing wrong?</p>