A006629 - OEIS

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A006629 as a simple table

n	a(n)
0	1
1	4
2	18
3	88
4	455
5	2448
6	13566
7	76912
8	444015
9	2601300
10	15426840
11	92431584
12	558685348
13	3402497504
14	20858916870
15	128618832864
16	797168807855
17	4963511449260
18	31032552351570
19	194743066471800
20	1226232861415695

[1,4,18,88,455,2448,13566,76912,444015,2601300, 15426840,92431584,558685348,3402497504, 20858916870,128618832864,797168807855, 4963511449260,31032552351570,194743066471800, 1226232861415695]