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This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.

A137398 Let S be a strictly monotonic sequence of length 2n and let p and q be subsequences of S each of length n such that the least element belongs to p and every element of S belongs to either p or q. The number of ways to select p such that for any index i the exchange of p(i) and q(i) makes at least one of p and q non-monotonic, is given by a(n).

Table of values

n a(n)
1 0
2 1
3 2
4 7
5 22
6 74
7 252
8 875
9 3078
10 10950
11 39316
12 142278
13 518364
14 1899668
15 6997688
16 25894579
17 96211398
18 358779118
19 1342323364
20 5037146738
21 18953759988
22 71497359884
23 270321915848
24 1024217489278
25 3888253473180
26 14787937448380
27 56337410614088
28 214967841333868
29 821473056041464
30 3143521372849960

List of values

[0, 1, 2, 7, 22, 74, 252, 875, 3078, 10950, 39316, 142278, 518364, 1899668, 6997688, 25894579, 96211398, 358779118, 1342323364, 5037146738, 18953759988, 71497359884, 270321915848, 1024217489278, 3888253473180, 14787937448380, 56337410614088, 214967841333868, 821473056041464, 3143521372849960]

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