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Published online by Cambridge University Press: 28 June 2013
Let $A, B$ be two square complex matrices of the same dimension
$n\leq 3$. We show that the following conditions are equivalent. (i) There exists a finite subset
$U\subset { \mathbb{N} }_{\geq 2} $ such that for every
$t\in \mathbb{N} \setminus U$,
$\exp (tA+ B)= \exp (tA)\exp (B)= \exp (B)\exp (tA)$. (ii) The pair
$(A, B)$ has property L of Motzkin and Taussky and
$\exp (A+ B)= \exp (A)\exp (B)= \exp (B)\exp (A)$. We also characterise the pairs of real matrices
$(A, B)$ of dimension three, that satisfy the previous conditions.