A037648 - OEIS

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A037648 as a simple table

n	a(n)
1	3
2	18
3	109
4	657
5	3942
6	23653
7	141921
8	851526
9	5109157
10	30654945
11	183929670
12	1103578021
13	6621468129
14	39728808774
15	238372852645
16	1430237115873
17	8581422695238
18	51488536171429
19	308931217028577
20	1853587302171462

[3,18,109,657,3942,23653,141921,851526,5109157, 30654945,183929670,1103578021,6621468129, 39728808774,238372852645,1430237115873, 8581422695238,51488536171429,308931217028577, 1853587302171462]