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{\bf Giovanni Lo Faro, Lorenzo Milazzo, Antoinette Tripodi}
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{\bf On the Upper and Lower Chromatic Numbers of \break 
BSQSs(16)}
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A mixed hypergraph is characterized by the fact that it possesses
${\cal C}$-edges as well as ${\cal D}$-edges. In a colouring of a mixed
hypergraph, every ${\cal C}$-edge has at least two vertices of the same
colour and every ${\cal D}$-edge has at least two vertices coloured
differently. The upper and lower chromatic numbers $\bar{\chi}$, $\chi$
 are the maximum and minimum numbers of
colours for which there exists a colouring using all the colours.
The concepts of mixed hypergraph, upper and lower chromatic numbers are
applied to $SQSs$.
In fact a BSQS is an SQS where all the blocks are
at the same time ${\cal C}$-edges and ${\cal D}$-edges.
In this paper we prove that any $BSQS(16)$ is colourable with the upper
chromatic
number $\bar{\chi}=3$ and we give new information about the
chromatic spectrum of BSQSs($16$).

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