On Fillmore's theorem over integrally closed domains

A well-known theorem of Fillmore says that if $A\in {\mathrm{M}}_n(K)$ is a non-scalar matrix over a field $K$ and ${\gamma }_1,\dots ,{\gamma }_n\in K$ are such that ${\gamma }_1+\cdots +{\gamma }_n=\mathrm{Tr}(A)$, then $A$ is $K$-similar to a matrix with diagonal $({\gamma }_1,\dots ,{\gamma }_n)$. Building on work of Borobia, Tan extended this by proving that if $R$ is a unique factorisation domain with field of fractions $K$ and $A\in {\mathrm{M}}_n(R)$ is non-scalar, then $A$ is $K$-similar to a matrix in ${\mathrm{M}}_n(R)$ with diagonal $({\gamma }_1,\dots ,{\gamma }_n)$. We note that Tan’s argument actually works when $R$ is any integrally closed domain and show that the result cannot be generalised further by giving an example of a matrix over a non-integrally closed domain for which the result fails. Moreover, Tan gave a necessary condition for $A\in {\mathrm{M}}_n(R)$ to be $R$-similar to a matrix with diagonal $({\gamma }_1,\dots ,{\gamma }_n)$. We show that when $R$ is a PID and $n\ge 3$, Tan’s condition is also sufficient.