The OEIS mourns the passing of Jim Simons and is grateful to the Simons Foundation for its support of research in many branches of science, including the OEIS.
login
The OEIS is supported by the many generous donors to the OEIS Foundation.

 

Logo
Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A271216 a(n) = 2^n floor(n/2)! 2
1, 2, 4, 8, 32, 64, 384, 768, 6144, 12288, 122880, 245760, 2949120, 5898240, 82575360, 165150720, 2642411520, 5284823040, 95126814720, 190253629440, 3805072588800 (list; graph; refs; listen; history; text; internal format)
OFFSET
0,2
COMMENTS
Number of symmetric rearrangement maps, i.e., rearrangement maps which satisfy a=a^(AI) and a^A = a^I.
REFERENCES
J. Burns, Counting a Class of Signed Permutations and Chord Diagrams related to DNA Rearrangement, Preprint.
LINKS
FORMULA
a(n) = 2^n floor(n/2)!
EXAMPLE
For n=0 the a(0)=1 solution is { ∅ }
For n=1 the a(1)=2 solutions are { +1, -1 }
For n=2 the a(2)=4 solutions are { +1+2, -2-1, +2+1, -1-2 }
For n=3 the a(3)=8 solutions are { +1+2+3, -3-2-1, +3-2+1, -1+2-3, +3+2+1, -1-2-3, +1-2+3, -3+2-1 }
MATHEMATICA
Table[2^n*Floor[n/2]!, {n, 0, 20}]
CROSSREFS
Sequence in context: A128055 A061285 A194810 * A102000 A165904 A192008
KEYWORD
nonn,easy
AUTHOR
Jonathan Burns, Apr 13 2016
STATUS
approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified May 24 06:28 EDT 2024. Contains 372772 sequences. (Running on oeis4.)