The OEIS is supported by the many generous donors to the OEIS Foundation.

 Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 59th year, we have over 358,000 sequences, and we’ve crossed 10,300 citations (which often say “discovered thanks to the OEIS”). Other ways to Give
 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A155519 a(n) = Sum (J(p): p is a permutation of {1,2,...,n}), where J(p) is the number of j <= ceiling(n/2) such that p(j) + p(n+1-j) = n+1. 2
 1, 2, 4, 16, 72, 432, 2880, 23040, 201600, 2016000, 21772800, 261273600, 3353011200, 46942156800, 697426329600, 11158821273600, 188305108992000, 3389491961856000, 64023737057280000, 1280474741145600000 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS a(n) = Sum_{k=0..ceiling(n/2)} k*A155517(n,k). LINKS FORMULA a(2n-1) = n(2n-2)!; a(2n) = 2(2n-2)!*n^2. EXAMPLE a(3)=4 because J(123)=2 (counting j=1,2), J(321)=2 (counting j=1,2) and J(132) = J(312) = J(213) = J(231) = 0. MAPLE a := proc (n) if `mod`(n, 2) = 1 then (1/2)*(n+1)*factorial(n-1) else (1/2)*factorial(n-2)*n^2 end if end proc: seq(a(n), n = 1 .. 23); CROSSREFS Cf. A155517, A155518. Sequence in context: A162119 A213327 A192623 * A180391 A275555 A058926 Adjacent sequences: A155516 A155517 A155518 * A155520 A155521 A155522 KEYWORD nonn AUTHOR Emeric Deutsch, Jan 26 2009 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.

Last modified November 30 02:32 EST 2022. Contains 358431 sequences. (Running on oeis4.)