# Finding the number of integral solutions to an equation given constraints.

I'm having a huge problem even understanding what to do here. Any guidance to get me going would be greatly appreciated.

If $n,k$ are positive integers, how many integral solutions are there to the equation $$x_1+x_2+ \cdots +x_k=n$$ if for all $i$, $x_i\geq2i$.

I've tried creating the equations for n=1 and on up, but that's not providing me any answers or patterns. Any help would be greatly appreciated. I think I need to create a weight function somehow, but even then I'm lost. Thank you.

Hint: We want to find the number of non-negative solutions of $$y_1+y_2+\cdots +y_k=n-2(1+2+\cdots+k).\tag{1}$$
For any non-negative integer solution of Rquation (1), if we then set $x_i=y_i+2i$, we obtain a solution of your constrained problem. And any solution of the constrained problem produces a solution of Equation (1).
Remark: The arithmetic may be a little nicer if we find the number of solutions of $z_1+z_2+\cdots+z_k =n-(1+3+5+\cdots +(2k-1))$ is positive integers.
• You are welcome. I left the "Stars and Bars" part (number of non-negative, or alternately number of positive) solutions of $w_1+\cdots+w_k=m$ to you, on the assumption that it is familiar. You could also go directly to generating functions without the preliminary reduction. But reduction to a familiar problem seems like the best way to go. – André Nicolas Nov 19 '13 at 20:42