A solid ring is a ring $R$ such that the multiplication
$R\otimes\_{\mathbb{Z}} R \to R$ is an isomorphism.  

These were classified by Bousfield and Kan; they are


1. subrings of $\mathbb{Q}$,
2. $\mathbb{Z}/n$,
3. products $R\times \mathbb{Z}/n$ with
$R\subseteq \mathbb{Q}$ and
every divisor of $n$ invertible in $R$
4. colimits of these.


I wonder how small the list gets if I put the additional
constraint that $\mathrm{Tor}\_{\mathbb{Z}}(R,R) = 0$.


REFERENCE:
Bousfield, A. K.; Kan, D. M.
The core of a ring.
J. Pure Appl. Algebra 2 (1972), 73–81.



Solid rings and Tor
Let $R^t$ be the torsion submodule and consider the exact sequence


$$0\rightarrow R^t\rightarrow R \rightarrow R/R^t\rightarrow 0$$


Bousfield and Kan show that the ring on the right is a localization of ${\mathbb Z}$, hence flat over ${\mathbb Z}$, so its $Tor$ with $R$ vanishes. Thus if we $Tor$ the above with $R$, we get $Tor(R^t,R)=Tor(R,R)$.


Now tensor the exact sequence with $R^t$ instead of $R$. This gives $Tor(R^t,R^t)=Tor(R^t,R)$. 


Thus $Tor(R,R)=Tor(R^t,R^t)$. But if $R^t$ is nonzero then (see Bousfield and Kan) it contains some ${\mathbb Z}/p{\mathbb Z}$ as a direct summand and hence $Tor(R^t,R^t)$ does not vanish. Thus $Tor(R,R)=0$ implies $R^t=0$. It follows (B/K 3.7) that $R$ is a localization of ${\mathbb Z}$.


