# summation formulas

Discussion in 'Math' started by jut, Sep 19, 2009.

1. ### jut Thread Starter Senior Member

Aug 25, 2007
224
2
This problem is in the context of discrete systems and signals.

My goal is to find an equation for y(n) without the summation.

$y(n)=\sum_k k(n-k)u(k-4)u(k+2-n)$

where $u(k)$ is the unit step function, k is from -inf to inf, and n=0,1,2,3...

So I need to eliminate the unit step functions from the equation by changing the limits of summation:

if $k-4\geq 0$ then $u(k-4)=1$. And so it's eliminated from my summation above.
if $k+2-n\geq 0$ then $u(k+2-n)=1$. And it's also eliminated.

So my limits of summation are $k\geq 4$ and $k\geq n-2$. But the limits don't define a finite range! Can anyone help?

Last edited: Sep 20, 2009