Absolutely Winning Bridge Hands

25th Dec 2019

11Protagoras

2jefftk

9Timothy Johnson

2jefftk

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4 comments, sorted by Click to highlight new comments since: Today at 6:09 AM

Your requirements are very slightly too strong. If you have more than 6 cards in a suit, the amount of them that have to be top cards is reduced. In your second example, a spade suit of A,K,Q,8,7,6,5,4,3,2 would have served just as well, as even if all the opposing spades were in one hand, playing out the A,K,Q would force them all out, making the remaining spades also winners.

I believe the number of solutions you get should be 12-choose-3 instead of 13-choose-4. After the number of cards in each of the first three suits is chosen, the number of cards in the fourth suit is already determined.

The usual explanation for this in a standard combinatorics class use the "stars and bars" method: https://en.wikipedia.org/wiki/Stars_and_bars_(combinatorics).

In bridge the best you can do is commit to take all the cards and then follow through. Even better if you do this without the help of a trump suit. Playing today I was curious: what fraction of hands are "absolutely winning" for 7NT, in that they'll let you make this contract no matter what your partner or opponents happen to hold?

Here's one example hand:

And here's a very different example:

Any absolutely winning 7NT hand needs to have all the aces, so that whatever your opponent leads you can take the first trick. Then all the other cards need to be "good", in that no other player can have a higher card in their suit.

Since a card being "good" requires that you also have every higher card in the same suit, all that matters for each suit is its length. If I tell you that an absolutely winning hand has four spades, you know they are A, K, Q, J. What we're doing, then, is assigning positive lengths to suits, where all lengths must sum to 13. But it's simpler if we ignore the aces, since you have to have all four, and assign non-negative lengths that add to 9. How many ways can we do this?

Since I'm more of a programmer than a mathematician, here's a way to solve this with code:

Which gives 715. [1] There are 52-choose-13 bridge hands (about 635B) so your chances of getting an absolutely winning 7NT hand is 715 in 635B or just a bit better than one in a billion.

While very unlikely, this is in the "it could happen, maybe" range and not all the way to "no way, unless the shuffle was rigged".

EDIT: the above is wrong in two ways. First, the number of spades is completely determined by the number of the other three suits, and so for

This gives 3,756 hands, or about six in a billion.`s in range(10 - c - d - h)`

should not have been included. And second, if you have seven or more of a suit then which lowest cards you have doesn't matter, because by the time you get to them no one else will have any of the suit. Here's a revised solution that fixes these two:[1] This is 13-choose-4, and while there's probably an obvious-in-retrospect reason why, I'm not seeing it.