I refer to generatingfunctionology.
On page 94, I see the use of $\delta_{n,0}$:
\begin{align*} mh(n,m) &= \sum_{r,m'\ge 1} h(n-rm', m-m')d_r & (n,m\ge 1; h(n,0) =\delta_{n,0}) \end{align*}
Similarly, on page 100, I see the use of $\delta_{0,k}$:
\begin{align*} \sum_{n,k\ge 0} p(n,k)x^ny^k &= \frac{1}{(1-yx)(1-yx^2)(1-yx^3)(1-yx^4)\cdots} \\ & (p(0,k) = \delta_{0,k}). \end{align*}
Both $\delta_{n,0}$ and $\delta_{0,k}$ don't seem to be defined in the book.
Is this a common notation? What (constant) values do $\delta_{n,0}$ and $\delta_{0,k}$ represent?