Multiplicative Semigroup of a Ring Let $I=\{0, 1, \ldots \}$ be the multiplicative semigroup of  non-negative integers. It is possible to find a ring $R$ such that the multiplicative semigroup of $R$ is isomorphic (as a semigroup) to $I$?
 A: Suppose that there is such an isomorphism, $\phi:R\to\mathbb N.$ In particular, such an $R$ would have to be commutative, since $\mathbb N$ is a commutative semigroup, and unital, since $1\in\mathbb N$ is an identity. Then, $-1\in R$ maps to some $\phi(-1)\neq 1\in\mathbb N.$ But $\phi(-1)^2=\phi((-1)^2)=\phi(1)=1,$ which is clearly impossible. Thus there cannot exist such a ring $R.$
A: Let $F$ be any field and take $R = F[x_1,\ldots,x_n]$ for any finite $n\ge 1$.  Every non-zero polynomial in $R$ factors uniquely into monic irreducibles times a constant in $F^\times$.
When $F = \mathbb F_2$, the factorization into monic irreducibles is exactly unique.  This gives $R \setminus \{0\}$ the structure of a free semigroup on countably many generators (all non-constant irreducible polynomials).
The structure of $I \setminus \{0\}$ is also the free semigroup on countably many generators (the primes).
It also seems like taking $F = \mathbb F_3$ should give the multiplicative semigroup of $\mathbb Z$.
