Disappearances in the Mediterranean

One passenger on a cruise in the Mediterranean Sea is disappearing after each day in port; there seems to be connection between the four of them. Cruise line security is beefed up, and Interpol is brought on board to investigate. Nothing unusual is found, although one of the investigators thinks he's found a mathematical connection. Professor Alfred Dunningham, in Paris at a math symposium, is toasted for his work in solving the Famous Sites Murders in Sydney, Australia. His phone rings, and on a conference call with Interpol, the ship's captain, and the cruise line CEO, they plead with him to join the ship and help them solve the mystery. He and his wife fly to meet and board ship, and given a suite on the 10th deck.  There is some similarity to the Sydney case, but the Professor dismisses that notion—Too obvious; would look like copy cat, he says. The cruise is almost over; will the Professor solve the case in time?