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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.

A099880 Number of preferential arrangements (or simple hierarchies) of 2*n labeled elements with two kinds of elements (where each kind has n elements).

Table of values

n a(n)
0 1
1 2
2 18
3 260
4 5250
5 136332
6 4327092
7 162309576
8 7024896450
9 344582629820
10 18890850749628
11 1144656941236536
12 75963981061424820
13 5479642938171428600
14 426894499408073653800
15 35720957482170932284560
16 3195135789350678836128450
17 304234032845362459798904220

List of values

[1, 2, 18, 260, 5250, 136332, 4327092, 162309576, 7024896450, 344582629820, 18890850749628, 1144656941236536, 75963981061424820, 5479642938171428600, 426894499408073653800, 35720957482170932284560, 3195135789350678836128450, 304234032845362459798904220]

Started in 1964 by Neil J. A. Sloane | Maintained by The OEIS Foundation Inc.

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