I learned something VERY interesting about playing cards!

[mjr]

Well, shoot, it's interesting to me, anyway.

If you have a standard deck of playing cards, there are 52! (approx 8.0658175e+67) permutations.

But...

If you take the cards, in order (A, 2, 3, etc...) and do perfect riffle shuffles (i.e. each stack has 26 cards, and each card successfully lays over top of ONE card), then in only eight riffle shuffles you will be back to the original order.

I read this, so I whipped up a quick little program to verify. And it does work.

[wolfie]

I read about that years ago. It takes 8 "out shuffles" (stack with original bottom card goes first, so outside cards remain the same) to put the deck back in order, but it takes 52 "in shuffles" (stack with original top card goes first, so original outside cards are now inside cards) to put it back in order.

[mjr]

I read about that years ago. It takes 8 "out shuffles" (stack with original bottom card goes first, so outside cards remain the same) to put the deck back in order, but it takes 52 "in shuffles" (stack with original top card goes first, so original outside cards are now inside cards) to put it back in order.

Yep. That's the one.

[Chromatix]

A variation of this fact was used in Liar Game, and is explained very nicely in Chapter 77. The game itself begins in Chapter 68.

[Jester]

A different variation on this idea is the fact that, when doing imperfect riffle shuffles, it takes only 7 shuffles to completely randomize a factory set deck.

What? I'm a magician. It's my job to know these things.