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For each 
$n\ge 4$ we construct a class of examples of a minimal 
$C$ -dependent set of 
$n$ automorphisms of a prime ring 
$R$ , where $C$ is the extended centroid of $R$ . For 
$n=4$ and 
$n=5$ it is shown that the preceding examples are completely general, whereas for 
$n=6$ an example is given which fails to enjoy any of the nice properties of the above example.
Let R be a prime ring with invoution *, of characteristic 0, with skew elements K and extended centroid C. Let a ∈ K be such that (ad a)n =0 on K. It is shown that one of the following possibilities holds: (a) R is an order in a 4-dimensional central simple algebra, (b) there is a skew element λ in C such that 
, (c) * is of the first kind, n ≡ 0 or n ≡ 3 (mod 4), and 
. Examples are given illustrating (c).
