A208654 - OEIS

%I #5 Mar 30 2012 18:58:13

%S 1,2,18,144,12000,540000,388962000,108425318400,650403212820480,

%T 1175952237465600000,57409367332363200000000,

%U 691636564481660937216000000,270540272566435932512004833280000

%N Number of palindromic paths through the subset array of {1,2,...,n}; see Comments.

%C A palindromic path through the subset array of {1,2,...,n} is essentially a palindrome using numbers i from {1,2,...n}, where the number of times i can be used in position k equals the multiplicity of i in the multiset of numbers in the k-element subsets of {1,2,...,n}.  See A208650 for a discussion and guide to related sequences.

%e For n=4, write

%e row 1:  1,2,3,4

%e row 2:  1,2; 1,3; 1,4; 2,3; 2,4; 3;4

%e row 3:  1,2,3; 1,2,4; 1,3,4; 2,3,4

%e row 4:  1,2,3,4

%e To form a palindromic path of length 4, there are 4 ways to choose 1st term from row 1, then 12 ways to choose 2nd term from row 2, then 3 ways to choose 3rd term, then 1 way to finish, so that a(4)=4*12*3*1=144.

%t m[n_] := Floor[(n + 1)/2]; z = 21;

%t g[n_] := Product[i*Binomial[n, i], {i, 1, m[n]}]

%t h[n_] := Product[Binomial[n - 1, i], {i, m[n], n - 1}]

%t Table[g[n], {n, 1, z}]   (* A208652 *)

%t Table[h[n], {n, 1, z}]   (* A208653 *)

%t Table[g[n] h[n], {n, 1, 2 z/3}]   (* A208654 *)

%t Table[g[n] h[n]/n, {n, 1, 2 z/3}] (* A208655 *)

%Y Cf. A208650, A208655.

%K nonn

%O 1,2

%A _Clark Kimberling_, Mar 02 2012