This site is supported by donations to The OEIS Foundation.



Annual Appeal: Please make a donation (tax deductible in USA) to keep the OEIS running. Over 5000 articles have referenced us, often saying "we discovered this result with the help of the OEIS".

(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A077612 Number of adjacent pairs of form (even,even) among all permutations of {1,2,...,n}. 3


%S 0,0,0,12,48,720,4320,60480,483840,7257600,72576000,1197504000,

%T 14370048000,261534873600,3661488230400,73229764608000,

%U 1171676233728000,25609494822912000,460970906812416000

%N Number of adjacent pairs of form (even,even) among all permutations of {1,2,...,n}.

%F a(n) = floor(n/2)*floor(n/2-1)*(n-1)!. Proof: There are floor(n/2)*floor(n/2-1) pairs (r, s) with r and s even and distinct. For each pair, there are n-1 places it can occur in a permutation and (n-2)! possible arrangements of the other numbers.

%F a(n) = A110660(n+2) * A000142(n-1). - _Michel Marcus_, Aug 29 2013

%o (PARI) a(n) = n\2 * (n\2-1)*(n-1)! ; \\ _Michel Marcus_, Aug 29 2013

%Y Cf. A077611, A077613.

%K nonn

%O 1,4

%A _Leroy Quet_, _Frank Ruskey_ and _Dean Hickerson_, Nov 11 2002

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

License Agreements, Terms of Use, Privacy Policy .

Last modified December 4 06:30 EST 2016. Contains 278749 sequences.