A Card-Dealing-Trick Sequence: Persistimis Possessiamo

Pete McCabe presented his trick, Persistimis Possessiamo, at the Gathering for Gardner in 2018.

Trick. Pete asked for two volunteers, let’s call them Alice and Bob. Bob took out his favorite card, the Queen of Spades, from the deck and put it back following Pete’s instructions. Then Alice dealt the deck alternatively into two piles, Bob’s and hers, starting with Bob’s. Alice took her pile and repeated the same process several times until only one card was left. And, abracadabra: it was Bob’s chosen Queen of Spades.

Pete McCabe is interested in scripting magic. In his blog post, Scripting Magic for Zoom, he describes ways to make sure that Bob inserts his card into the 22nd place without using sleight of hand, but rather using a theatrical script which makes the process magical rather than mathematical. The magic part is related to the fact that the number of letters in the trick title, Persistimis Possessiamo, is 22. As a result, he can do the trick on Zoom without ever touching the cards.

Once a magician knows how to manipulate the volunteer to insert the card into a specific place in the deck, the trick becomes deterministic and works on any-sized deck, as long as the magician can calculate where the card goes. We will now perform this calculation.

We denote our card-inserting sequence as a(n), where n is the size of the deck, and a(n) is the place where the card is inserted. For starters, a(2n+1) = a(2n): when the size of the deck is odd, the last card during the first deal goes to Bob, and doesn’t effect the other deals. Now, we obviously have a recursion. First, we observe that in order to end up in Alice’s pile after the first deal, Bob’s chosen card should occupy an even-numbered place. Suppose we start with 2n cards. After the first deal, Bob’s chosen card is in the place number a(2n)/2 from the bottom in Alice’s pile. That means, the card is in the place number n + 1 − a(2n)/2 from the top. This gives us an equation: a(n) = n + 1 − a(2n)/2, which is equivalent to a recursion: a(2n) = 2(n + 1 − a(n)).

Given that each element of the sequence a(n) is doubled, we are only interested in even-indexed values. Consider b(n) = a(2n) = a(2n+1). Then b(1) = 2, and the recursion for b is b(n) = 2(n + 1 − b⌊n/2⌋).

From here, we get the sequence, which is now sequence A350652 in the OEIS:

2, 2, 4, 6, 8, 6, 8, 6, 8, 6, 8, 14, 16, 14, 16, 22, 24, 22, 24, 30, 32, 30, 32, 22, 24, 22, 24, … .


Leave a comment