= Solution
{c}
A <multiplicative subset> $S$ is a <left Ore set> if for every $s\in S$ and $r\in R$ there are $s'\in S$ and $r'\in R$ with
$$
\boxed{s'r=r's.}
$$
Equivalently, the subsets $Sr$ and $Rs$ intersect for each such pair. This is the common-left-multiple condition needed for <Ore localization> with denominators on the left. In a <noncommutative domain>, no extra condition concerning annihilation by denominators is needed, since all members of $S$ are <regular elements of a ring>.
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