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A deck of playing cards is randomized by a procedure called shuffling to provide an element of chance in card games. There are several techniques for this; the most common is called a "riffle", in which half of the deck is held in each hand with the thumbs inward, then cards are released by the thumbs so that they fall to the table intertwined.
This can also be done by placing the halves flat on the table with their rear corner touching, then lifting the back edges with the thumbs while pushing the halves together. While this method is a bit more difficult, it is often used in casinos because it minimizes the risk of exposing cards during the shuffle.
Another procedure is called "stripping", where small groups of cards are removed from the top or bottom of a deck and replaced on the opposite side (or just assembled on the table in reverse order). This is a much less effective randomizing procedure, and is not recommended unless used in conjuction with riffling.
"Pushing" is the procedure of pushing the ends of two halves of a deck against each other in such a way that they naturally intertwine. This requires skill and practice, as does "fanning", which involves spreading the halves into fan shapes and intertwining them.
Shuffling is often followed by a cut.
The mathematician and magician Persi Diaconis is an expert on the theory and practice of card shuffling, and an author of a famous paper on the number of shuffles needed to randomize a deck. Recently, the work of Trefethen et al. has questioned some of Diaconis' results. The difference appears to hinge on the definition of "randomize".
References:
- D. Aldous and P. Diaconis, Shuffling cards and stopping times, American Mathematical Monthly 93 (1986), 333-348.
- Trefethen, L. N. and Trefethen, L. M. How many shuffles to randomize a deck of cards? Proceedings of the Royal Society London A 456, 2561 - 2568 (2000)
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