Ironically seven perfectly interleaved riffle shuffles will return a deck to its original order, so the title is spectacularly wrong for one famous result.

Also the new result is cool! (14 semi bad riffle shuffles are sufficient to mix)

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zahlman 2 days ago | parent | next [-]

It requires eight perfect riffle shuffles, not seven. (I just checked at the Python REPL.) And actually it depends on whether the riffles are done "in" or "out" (i.e. which half of the deck the new top card comes from).

I had understood that seven "typical" riffle shuffles produce good randomness.

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WillAdams 2 days ago | parent [-]

Yeah, my father could do that consistently, and used it to teach me "Always cut cards" having also memorized the card order for each shuffle --- I guess being on a troop ship for weeks on end had to have some sort of up-side.

Nick Scarne is an interesting name to look up, and his writings are almost on a level with his facility to manipulate cards.

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pmarreck 2 days ago | parent [-]

I love the fact that he intended to become a card shark but his mom steered him towards magic instead, and then he got hired by the US Army to teach enlistees about the various scams they might encounter overseas, in addition to serving as the "hands" for Paul Newman in the movie The Sting, which was about conning a mob boss? Damn, "interesting" is apt!

https://en.wikipedia.org/wiki/John_Scarne

Whenever someone acquires a morally-neutral skill (like "card manipulation" or "martial arts") that can be used for "good" or "evil" and chooses the former, that's almost always a good story... especially if they flirted with the latter...

	▲	WillAdams 20 hours ago \| parent [-]
		Thanks for correcting the name and the link --- not sure where I got "Nick" from...

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PaulHoule 2 days ago | parent | prev [-]

Well I'd imagine a "perfect" shuffling procedure would have an equal probability of all 52! possible outputs which includes the original input and one would expect the sequence that gets you there would be highly symmetric.