The OEIS is supported by the many generous donors to the OEIS Foundation.
| n | a(n) |
|---|---|
| 1 | 1 |
| 2 | 2 |
| 3 | 4 |
| 4 | 18 |
| 5 | 60 |
| 6 | 252 |
| 7 | 1860 |
| 8 | 9552 |
| 9 | 59616 |
| 10 | 565920 |
| 11 | 4051872 |
| 12 | 33805440 |
| 13 | 465239808 |
| 14 | 4294865664 |
| 15 | 35413136640 |
| 16 | 768372168960 |
| 17 | 8757710173440 |
| 18 | 79772814777600 |
| 19 | 1986906367584000 |
| 20 | 22082635812268800 |
| 21 | 280886415019776000 |
| 22 | 7683780010315046400 |
[1,2,4,18,60,252,1860,9552,59616,565920,4051872, 33805440,465239808,4294865664,35413136640, 768372168960,8757710173440,79772814777600, 1986906367584000,22082635812268800, 280886415019776000,7683780010315046400] |