  Because of unmatched improvements in CPU performance, memory transfers have
become a bottleneck of program execution. As discovered in recent years, this
also affects sorting in internal memory. Since partitioning around several
pivots reduces overall memory transfers, we have seen renewed interest in
multiway Quicksort. Here, we analyze in how far multiway partitioning helps in
Quickselect.
  We compute the expected number of comparisons and scanned elements
(approximating memory transfers) for a generic class of (non-adaptive) multiway
Quickselect and show that three or more pivots are not helpful, but two pivots
are. Moreover, we consider "adaptive" variants which choose partitioning and
pivot-selection methods in each recursive step from a finite set of
alternatives depending on the current (relative) sought rank. We show that
"Sesquickselect", a new Quickselect variant that uses either one or two pivots,
makes better use of small samples w.r.t. memory transfers than other
Quickselect variants.
