Solution
= Solution
A lambda term $F$ lambda-defines $f:\mathbb N^k\to\mathbb N$ when, for all $n_1,\ldots,n_k$,
$$
F\,c_{n_1}\cdots c_{n_k}\equiv_\beta c_{f(n_1,\ldots,n_k)},
$$
where $c_n$ is the <Church numeral> for $n$.
= Solution
A lambda term $F$ lambda-defines $f:\mathbb N^k\to\mathbb N$ when, for all $n_1,\ldots,n_k$,
$$
F\,c_{n_1}\cdots c_{n_k}\equiv_\beta c_{f(n_1,\ldots,n_k)},
$$
where $c_n$ is the <Church numeral> for $n$.