| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 20 |
| 3 | 104 |
| 4 | 556 |
| 5 | 3032 |
| 6 | 16778 |
| 7 | 93872 |
| 8 | 529684 |
| 9 | 3008864 |
| 10 | 17184188 |
| 11 | 98577712 |
| 12 | 567591142 |
| 13 | 3278348608 |
| 14 | 18986482250 |
| 15 | 110217131168 |
| 16 | 641125473092 |
| 17 | 3736134109936 |
| 18 | 21807240851480 |
| 19 | 127469052615104 |
| 20 | 746057665449076 |
| 21 | 4371699398312704 |
| 22 | 25644387465768860 |
[1,4,20,104,556,3032,16778,93872,529684,3008864, 17184188,98577712,567591142,3278348608, 18986482250,110217131168,641125473092, 3736134109936,21807240851480,127469052615104, 746057665449076,4371699398312704, 25644387465768860] |