Công thức nào sau đây là chính xác

$y(n)=x(n)*h(n)={\displaystyle \sum _{k=-\infty }^{n}x(k)h(k-n)}$

$y(n)=x(n)*h(n)={\displaystyle \sum _{k=-N}^{n}x(k)h(n-k)}$

$y(n)=x(n)*h(n)={\displaystyle \sum _{k=-\infty }^{n}x(k)h(n-k)}$

$y(n)=x(n)*h(n)={\displaystyle \sum _{k=-\infty }^{n}x(k)h(n-k)}$