FBI Agent Alice is hot on the trail of computer hacker Bob, who is hiding
in one of 17 caves.

The caves are arranged in a line, and every night Bob moves from the cave
he is in to one of the caves on either side of it.

Alice can search two caves each day, with no restrictions on her choice.
For example, if Alice searches (1 2), (2 3), ... , (16 17), then she is
certain to catch Bob, though it might take her 16 days.

What is the shortest time in which Alice can be guaranteed of catching
Bob?

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Midshipmen submitting the shortest times (with explanations) by noon on
Wednesday, September 27, win a cookie. The best solution will be posted on
the problem bulletin board.
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A solution is "shortest" if it explains how Alice is guaranteed to find
Bob and no one else submits a shorter time. A solution is "best" if it
includes the clearest correct explanation. Submit solutions to Prof. Hanna
at mathprob@usna.edu (please no attachments!) or via the mailbox in
Chauvenet 301.

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Correct solutions to problem #113 came from Midn. 4/c Foster and
Greenawalt. Mr. Foster's solution is posted on the bulletin board in the
Chauvenet lab deck corridor. There are 7 planes equidistant from any 4
points which don't all lie in one plane. This was Macalester Problem of
the Week #909.