A187817 - cp's OEIS frontend

A187817 Number of permutations p of {1,...,n} such that exactly two elements of {p(1),...,p(i-1)} are between p(i) and p(i+1) for all i from 3 to n-1.

n	a(n)
0	1
1	1
2	2
3	6
4	4
5	4
6	4
7	4
8	8
9	12
10	20
11	32
12	52
13	104
14	188
15	344
16	616
17	1116
18	2232
19	4236
20	8084
21	15212
22	28760
23	57520
24	111512
25	216804
26	417560
27	806440
28	1612880
29	3162132
30	6209192
31	12113136
32	23670168
33	47340336
34	93411704
35	184494460
36	362693224
37	713767712
38	1427535424

[1, 1, 2, 6, 4, 4, 4, 4, 8, 12, 20, 32, 52, 104, 188, 344, 616, 1116, 2232, 4236, 8084, 15212, 28760, 57520, 111512, 216804, 417560, 806440, 1612880, 3162132, 6209192, 12113136, 23670168, 47340336, 93411704, 184494460, 362693224, 713767712, 1427535424]

cp's OEIS Frontend

A187817 Number of permutations p of {1,...,n} such that exactly two elements of {p(1),...,p(i-1)} are between p(i) and p(i+1) for all i from 3 to n-1.

Table of values

List of values