A018957 - OEIS

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A018957 as a simple table

n	a(n)
2	1
3	12
4	18
5	213
6	868
7	6811
8	40267
9	283800
10	1893581
11	13245343
12	92117860
13	651848468
14	4635906047
15	33287875110
16	240521004903

[1,12,18,213,868,6811,40267,283800,1893581, 13245343,92117860,651848468,4635906047, 33287875110,240521004903]