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A332758
Number of fixed-point free involutions in the n-fold iterated wreath product of C_2.
3
0, 1, 3, 17, 417, 206657, 44854599297, 2021158450131287670017, 4085251621720569336520310526902208564886017, 16689280870666586360302304039420036318743515355074220606298783584912362351240766944257
OFFSET
0,3
COMMENTS
Also the number of fixed-point free involutions in a fixed Sylow 2-subgroup of the symmetric group of degree 2^n.
Also the number of fixed-point free involutory automorphisms of the complete binary tree of height n.
FORMULA
a(n) = a(n-1)^2 + 2^(2^(n-1)-1), a(0) = 0.
a(n) ~ C^(2^n) for C = 1.467067423065535412629251121186749718727038915553188083467...
EXAMPLE
For n=2, the a(2)=3 fixed-point free involutions in C_2 wr C_2 (which is isomorphic to the dihedral group of degree 4) are (12)(34), (13)(24), and (14)(23).
MATHEMATICA
Nest[Append[#1, #1[[-1]]^2 + 2^(2^(#2 - 1) - 1)] & @@ {#, Length@ #} &, {0}, 9] (* Michael De Vlieger, Feb 25 2020 *)
CROSSREFS
Cf. A332757.
Sequence in context: A349643 A098138 A009719 * A023150 A257116 A305375
KEYWORD
nonn
AUTHOR
Nick Krempel, Feb 22 2020
STATUS
approved