Seven Perfect Shuffles Randomize a Deck of Cards. But How Many Sloppy Ones? Introduction In 1992, mathematicians famously proved that seven “riffle shuffles” — the kind where a player splits a deck of cards into two piles, then uses their thumbs to interleave them back together in a zipperlike motion — are enough to mix up the deck. When Dave Bayer and Persi Diaconis came up with this proof, they ...