I am interested in the collection of possible values for permanents over square binary matrices. Consider $n \times n$ 0-1 matrices. The possible permanents for $n=1$ are $\{0,1\}$. For $n=2$ the possible permanents are $\{0,1,2\}$, and for $n=3$ the permanents are $\{0,1,2,3,4,6\}$. Note that $5$ is missing for $n=3$.

Let $p(n)$ denote the set of permanents for an $n \times n$ binary matrix, and let $$p(\mathbb{N}) = \bigcup_{n \in \mathbb{N}} p(n)$$ We can show at least that $n! \in p(\mathbb{N})$ for all $n$ and that $p(\mathbb{N})$ is closed under products.

Is this set $p(\mathbb{N})$ well understood? A pointer would be welcome. If $\mathbb{N} \subset p(\mathbb{N})$, a construction proof would be appreciated. It is not clear how to extend the trivial rectangular matrix with permanent $k$ to a square matrix.

The following related integer sequence counts distinct permanents: http://oeis.org/A087983