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A373416
Fixed points of A323712: sizes k of card decks that return to original order after k pile shuffles (but not earlier).
2
1, 3, 4, 6, 9, 12, 22, 27, 28, 36, 46, 52, 60, 70, 78, 81, 100, 102, 148, 166, 172, 180, 190, 196, 198, 238, 243, 262, 268, 270, 292, 310, 316, 348, 358, 366, 372, 382, 388, 420, 460, 462, 478, 486, 502, 508, 540, 556, 598, 606, 612, 646, 652, 660, 676, 700, 708, 718, 729, 742
OFFSET
1,2
COMMENTS
The pile shuffle of a card stack [1,2,3...,n] puts 1 at the bottom of a first auxiliary stack, 2 at the bottom of a second auxiliary stack, 3 above the 1, 4 above the 2, 5 above the 3, 6 above the 4.. eventually the shuffled stack is the first auxiliary stack on top of the second auxiliary stack.
Sequence A263458 contains sizes 2k of card decks that return to reverse order after k pile shuffles. They return to original order after 2k shuffles (2 reversals), so entries of A263458 are candidates for entries here. The case of 30 cards shows that this is not that simple: 30 cards are in reverse order after 5 shuffles, back to original order after 10 shuffles, in reverse order after 15 shuffles (so 30 is in A263458) etc. Because the return to normal order happens already after 10 shuffles (not only at 30), 30 is not in this sequence here.
FORMULA
Equals the ordered set {k: A323712(k) = k}.
CROSSREFS
Cf. A323712, A163781 (even terms halved ?), A263458.
Sequence in context: A241639 A241655 A288346 * A078743 A096846 A140570
KEYWORD
nonn
AUTHOR
R. J. Mathar, Aug 02 2024
STATUS
approved