title: How to define a general function in mathematica?
body: <p>How can I define a general function without the exact expression in Mathematica? For example, I don't need this:</p>
  
  <pre><code>a[x_, y_]:= 2x + 3y,
  </code></pre>
  
  <p>I need to work with a general parameter a(x,y) instead:</p>
  
  <pre><code>a[x_, y_]:= a[x, y].
  </code></pre>
  
  <p>Of course, this line doesn't work too well.</p>
  
  <p>Edit1:</p>
  
  <p>Well, I am using the parameter in another unknown function (unknown because it is defined only in the package), which works basically as a derivative, i.e. it's a very complicated mix of first and second derivatives.</p>
  
  <p>This line</p>
  
  <pre><code>a[x_, y_]:= a[x, y],
  </code></pre>
  
  <p>actually works (kind of), because the parameter 'survives' the package as a function, except in the output I get the annoying "<code>Hold[a[x,y]]</code>" instead of just <code>a[x,y]</code>, and I can't use the result as a function anymore. </p>