As Jon said, the &lt;authorization&gt; node should not be a child of the &lt;customErrors&gt; node.

Unrecognized element 'authentication'.
I am very confused with complete induction. Because in every task there is something different to do, and I never know what to insert (thats my biggest problem). 
Here's the example:
Proof with complete induction. Please please help me, because I have exams coming up (I am just becoming a primary school teacher..)


  For $n\in\mathbb{N}$:
  
  $$\sum^n_{i=1}\frac{1}{(2i-1)(2i+1)}=\frac{n}{2n+1}$$

