Dijkstra was right – recursion should not be difficult
Anything  that is self-referential is hard.  I remember when we covered proof-by-induction in HS math.  There was a significant fraction of the class that did not believe a given proof was sufficient.<p>If you can&#x27;t assure yourself of the correctness of a fairly simple recursive function that is given to you, then composing one yourself is going to be nigh impossible.<p>[edit]<p>From the article: <i>Instead trust that it returns correct sum for all elements of an array a[i]</i><p>This is exactly the point where many people get hung up.  They cannot proceed to reading the next line until they understand how the recursive call works.  They are unwilling to trust that the function works as advertised, and cannot shift gears to continue past that.  In my experience, telling them to &quot;just trust it&quot; and such seems to not help much.