## Monday, February 8, 2021

Since the shutdowns of the spring, I’ve been playing more tennis, with my son and with graduate students. Sometimes you end up having three people wanting to play tennis, though, and what do you do?

The canned solution is Canadian doubles where you have two people on one side and one on the other, you get two points for winning on the singles side and one for winning on doubles, you rotate the players counterclockwise between games, and you end at some fixed number of points, say 11, by a lead of two. And the singles court is used on the singles side while the doubles court is used on the doubles side.

This is a good game: alternating between being a single playing facing off against two and playing as part of a team is fun. However, we noticed two difficulties. First, the standard rotation scheme has the result that each time one serves, one faces the same person. That reduces the variation. A fix of this is to depart from the counterclockwise rotation. A more serious problem is that towards the end of a match, when playing on the double side, one can have a perverse desire to lose the game. For imagine that your partner has 10 points, you have 8, and the third player has less than 8. Then if your side wins, your partner reaches 11 and wins the match. But if your side loses, you may still have a chance to win the match later. This can sap motivation.

Also missing from the standard Canadian doubles is that in the interest of trying all combinations, it would be nice to have the chance to be a singles server and a singles receiver in the same match.

So, after a number of iterations, here is improved Canadian doubles (I am Canadian, by the way). Instead of playing to a fixed score, you play three rounds of six games. Highest score wins. Ties are possible. You can end early if you can see that the number of games left is insufficient to change the ranking between the players.

The first and third rounds have serving from the doubles side. The second round has serving from the singles side. In each round, positions rotate in such a way that each of the six arrangements occurs once. Moreover, the positions are so arranged as to minimize the same player “being on the spot” too many times in a row. Thus, no one serves twice in a row or is in singles twice in a row, and we rule out the tiring sequence of singles, then serving, then singles again. I generated the sequences with a brute force python script.

Round 1: Service from doubles side

B C A
C A B
A B C
C B A
A C B
B A A

Round 2: Service from singles side

A C B
C A B
A B C
B A C
C B A
B C A

Round 3: Service from doubles side