A115455 - cp's OEIS frontend

A115455 a(n) = number of reverse alternating fixed-point-free involutions w on 1,2,...,2n, i.e., w(1) < w(2) > w(3) < w(4) > ... < w(2n), w^2=1 and w(i) != i for all i.

n	a(n)
0	1
1	0
2	1
3	1
4	4
5	13
6	59
7	308
8	1871
9	12879
10	99144
11	843735
12	7865177
13	79698760
14	872235089
15	10253148625
16	128839087676
17	1723418002261
18	24450430660739
19	366702601116524
20	5796979684239647
21	96339860422218143
22	1679159568980521104
23	30628034488033962287

[1, 0, 1, 1, 4, 13, 59, 308, 1871, 12879, 99144, 843735, 7865177, 79698760, 872235089, 10253148625, 128839087676, 1723418002261, 24450430660739, 366702601116524, 5796979684239647, 96339860422218143, 1679159568980521104, 30628034488033962287]

cp's OEIS Frontend

A115455 a(n) = number of reverse alternating fixed-point-free involutions w on 1,2,...,2n, i.e., w(1) < w(2) > w(3) < w(4) > ... < w(2n), w^2=1 and w(i) != i for all i.

Table of values

List of values