来自在线整数百科全书的问候语!http://oeis.org/ Search: id:a277083 Showing 1-1 of 1 %I A277083 %S A277083 1,1,1,1,1,2,1,1,2,3,4,3,2,1,1,8,36,120,322,728,1428,2472,3823,5328, %T A277083 6728,7728,8092,7728,6728,5328,3823,2472,1428,728,322,120,36,8,1,1,8, %U A277083 84,504,3178,15512,74788,311144,1252819,4577328,16087512,52691408,165911284 %N A277083 Irregular triangle read by rows: T(n,k) = number of size k subsets of S_n that remain unchanged by a rotation of 180 degrees. %C A277083 A permutation, p, can be thought of as a set of points (i, p(i)). 如果你绘制所有的点并将图片旋转180度,那么你就得到一个排列。{%A77083T(n,k)是Syn的大小k个子集的数目,它保持180度的旋转不变。- R(n), i ) * binomial( R(n), k-2*i ) for i in [0..floor(k/2)] ) where R(n) = A037223(n). %e A277083 For n = 3 and k = 3, the subsets unchanged by rotating 180 degrees are {213,132,123}, {231,312,123}, {321,132,213} and {321,231,312} so T(3,3) = 4. %e A277083 Triangle starts: %e A277083 1, 1; %e A277083 1, 1; %e A277083 1, 2, 1; %e A277083 1, 2, 3, 4, 3, 2, 1; %Y A277083 Row lengths give A038507. %Y A277083 Cf. A037223. %K A277083 nonn,tabf %O A277083 0,6 %A A277083 _Christian Bean_, Sep 28 2016 # Content is available under The OEIS End-User License Agreement: http://oeis.org/LICENSE