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%I A051732 #18 Nov 11 2017 12:53:12
%S A051732 1,1,2,2,3,5,6,6,4,9,4,28,10,9,14,12,5,70,18,24,10,7,210,126,110,60,
%T A051732 26,120,9,29,30,60,6,33,308,42,60,990,30,374,27,41,60,2618,840,840,
%U A051732 420,1386,24,15,50,644,840,53,18,1386,14,13300,2520,1260,55,6930,50,60,7
%N A051732 Number of rounds of shuffling required to restore a deck of n cards to its original order: shuffling is done by keeping first card, putting second at end of deck, keeping next, putting next at end and so on.
%C A051732 From _Andrew Howroyd_, Nov 11 2017:(Start)
%C A051732 The shuffling process is the same as the 'deal one, skip one' method described in A289386 except that dealt cards are placed face up. With this variation the first card always remains the first card.
%C A051732 Equivalently, place the numbers 1..n-1 on a circle and cyclically mark the 2nd unmarked number until all numbers are marked. The sequence in which the numbers are marked defines a permutation. The order of this permutation is a(n). The numbers 1..n can also be used, but in that case the number 1 should be marked first.
%C A051732 (End)
%H A051732 Andrew Howroyd, <a href="/A051732/b051732.txt">Table of n, a(n) for n = 1..2000</a>
%H A051732 Gary Antonick, <a href="http://wordplay.blogs.nytimes.com/2016/04/25/finkel-polka-dot/">The Polka Dot Puzzle</a>, NY Times Wordplay blog.
%F A051732 a(2^n+1) = n+1. - Ripatti A. (ripatti(AT)inbox.ru), Feb 04 2010
%F A051732 a(A163782(n)+1) = A163782(n). - _Andrew Howroyd_, Nov 11 2017
%e A051732 From _Andrew Howroyd_, Nov 11 2017: (Start)
%e A051732 a(6) = 5 because it takes 5 rounds of shuffling to return the cards to their original order as illustrated below:
%e A051732 1 2 3 4 5 6
%e A051732 1 3 5 2 6 4
%e A051732 1 5 6 3 4 2
%e A051732 1 6 4 5 2 3
%e A051732 1 4 2 6 3 5
%e A051732 1 2 3 4 5 6
%e A051732 (End)
%o A051732 (PARI)
%o A051732 P(n,i)={my(d=2*n+1-2*i); while(d<n, d*=2); 2*n-d}
%o A051732 Follow(s,f)={my(t=f(s),k=1); while(t>s, k++; t=f(t)); if(s==t, k, 0)}
%o A051732 CyclePoly(n,x)={my(q=0); for(i=1, n, my(l=Follow(i,j->P(n,j))); if(l,q+=x^l)); q}
%o A051732 a(n)={my(q=CyclePoly(n,x), m=1); for(i=1,poldegree(q),if(polcoeff(q,i), m=lcm(m,i))); m} \\ _Andrew Howroyd_, Nov 11 2017
%Y A051732 Cf. A163782, A289386.
%K A051732 easy,nonn,nice
%O A051732 1,3
%A A051732 Marie-Christine Haton (Marie-Christine.Haton(AT)loria.fr)
%E A051732 Name clarified by _Andrew Howroyd_, Nov 11 2017

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