The convolution of the functions $$f(t)$$, $$g(t)$$ (interpreted on $$]-\infty,\infty[$$) is defined as
$$(f * g)(t)=\int_{-\infty}^{\infty} f(t)g(x-t)dt$$
$$(f * g)(n) = \sum_{k \in D} f(k) g(n - k)$$