Printing a diamond of numbers
using namespace std is not preferred, although not that bad for small programs.
Each variable should be declared/initialized on separate lines.  This improves readability and also makes it easier to add any necessary comments.

unsigned i;
unsigned k = 0;
unsigned n;


Same with this:

cout &lt;&lt; "n= ";
cin &gt;&gt; n;

Prefer "\n" to std::endl here (the latter flushes the buffer, which takes longer).  It's still okay to use the latter where both flushing and newlining are needed.
Always use descriptive names for variables.  Single-characters are best for loop counters (such as i).  This will vastly improve readability as you won't need comments to describe them.
It looks like you could use recursion instead of all these loops.  It may take longer (if you input a large number), but it should at least simplify the logic.