Enumeration Solitaire is a one-player card game with a two-sentence rule set and a genuinely nontrivial answer. You shuffle a deck, deal cards face up, and count "ace, two, three, ..., king" over and over as you go. If the rank you speak ever matches the rank of the card you just dealt, you lose. Get through all 52 cards without a match and you win.

It is a four-copy generalization of the classical probleme des rencontres (the derangement problem), and the solution is one inclusion-exclusion argument compressed into one generating function.

Our working notes: Enumeration Solitaire.

The Problem

This one …

Baby Poker is a two-player dice game that fits on a napkin and still manages to be a real bluffing game. Each player rolls a single die concealed in a cup. There is one round of betting. That is it. And yet the optimal strategy involves mixed frequencies, a value range, a bluffing range, and a call/fold cutoff. It is the smallest game I know of that has all the moving parts of full poker.

Our working notes: Baby Poker.

The Problem

This one comes from the FiveThirtyEight Riddler column (now defunct, but happily still on the Wayback Machine …