Mathematics
The number of in shuffles required to return a deck of cards of even size N, to original order is given by the multiplicative order of 2 modulo (N + 1).
For example, for a deck size of N = 2, 4, 6, 8, 10, 12 ..., the number of in shuffles needed are: 2, 4, 3, 6, 10, 12, 4, 8, 18, 6, 11, ... (sequence A002326 in OEIS).
For a standard deck of 52 playing cards, the number of in shuffles required to return the deck to its original order is 52.
Read more about this topic: In Shuffle
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