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A019567 Order of the Mongean shuffle permutation of 2n cards: a(n) is least number m for which either 2^m + 1 or 2^m - 1 is divisible by 4n + 1. 4
1, 2, 3, 6, 4, 6, 10, 14, 5, 18, 10, 12, 21, 26, 9, 30, 6, 22, 9, 30, 27, 8, 11, 10, 24, 50, 12, 18, 14, 12, 55, 50, 7, 18, 34, 46, 14, 74, 24, 26, 33, 20, 78, 86, 29, 90, 18, 18, 48, 98, 33, 10, 45, 70, 15, 24, 60, 38, 29, 78, 12, 84, 41, 110, 8, 84, 26, 134, 12, 46, 35, 36, 68, 146 (list; graph; refs; listen; history; text; internal format)
OFFSET
0,2
COMMENTS
Write down 1, then 2 to left, 3 to right, 4 to left, ..., getting [ 2n,2n-2,...,4,2,1,3,5,...,2n-1 ]; the sequence 2,3,6,4,6,10,14,5,18,10,12,21,26,9,... gives order of permutation sending 1 to 2n, 2 to 2n-2, ..., 2n to 2n-1.
Equivalently, the sequence 2,3,6,4,6,10,14,5,18,10,12,21,26,9,... gives the number of Mongean shuffles needed to return a deck of 2n cards (n=1,2,3,...) to its original order.
It appears that a(n) = order((-1)^(n+1)*2 in Z_{2n+1}) / f with f=1 when n==2 (mod 3) and for n = 0, 19, 21, 30,33, 52, 55, 61, 63, 70, ..., f=2 else. I don't know how to characterize the "exceptional" n's. - M. F. Hasler, Mar 31 2019
REFERENCES
A. P. Domoryad, Mathematical Games and Pastimes, Pergamon Press, 1964; see pp. 134-135.
W. W. Rouse Ball, Mathematical Recreations and Essays, 11th ed. 1939, p. 311
LINKS
P. Diaconis, The mathematics of perfect shuffles, Adv. Appl. Math. 4 (2) (1983) 175-196.
Arne Ledet, The Monge shuffle for two-power decks, Math. Scand. Vol 98, No 1 (2006), 5-11.
FORMULA
a(A163777(n)/2) = A163777(n). - Andrew Howroyd, Nov 11 2017
EXAMPLE
Illustrating the initial terms:
n 4n+1 2^m+1 2^m-1 m
0 1 1 1
1 5 5 2
2 9 9 3
3 13 5*13 6
4 17 17 4
5 21 3*21 6
6 25 41*25 10
MAPLE
A019567:= proc(n)
for m from 1 do
if modp(2^m-1, 4*n+1) =0 or modp(2^m+1, 4*n+1)=0 then
return m ;
end if;
end do;
end proc: # N. J. A. Sloane, Jul 28 2007
MATHEMATICA
a[n_] := For[m=1, True, m++, If[AnyTrue[{-1, 1}, Divisible[2^m+#, 4n+1]&], Return[m]]];
Table[a[n], {n, 0, 100}] (* Jean-François Alcover, Aug 26 2019 *)
PROG
(PARI) A019567(n, z=Mod(2, 4*n+1))=for(m=1, oo, bittest(5, lift(z^m+1))&&return(m)) \\ M. F. Hasler, Mar 31 2019
CROSSREFS
Sequence in context: A349381 A156688 A348243 * A098286 A226615 A138608
KEYWORD
nonn,easy
AUTHOR
John Bullitt (metta(AT)world.std.com), N. J. A. Sloane and J. H. Conway
EXTENSIONS
Comments corrected by Mikko Nieminen, Jul 26 2007, who also provided the Domoryad reference
Definition edited by N. J. A. Sloane, Nov 09 2017
STATUS
approved

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Last modified March 18 22:56 EDT 2024. Contains 370952 sequences. (Running on oeis4.)