A086405-OEIS公司

A086405号作为一个简单的表

n个	a（n）
0	1
1	4
2	18
三	84
4	396
5	1872
6	8856
7	41904
8	198288
9	938304
10	4440096
11	21010752
12	99423936
13	470479104
14	2226331008
15	10535111424
16	49852682496
17	235905426432
18	1116316463616
19	5282466223104
20	24996898556928
21	118286594002944

[1,4,18,84,396,1872,8856,41904,198288,938304,4440096,21010752,99423936,470479104,2226331008,10535111424,49852682496,235905426432,1116316463616,5282466223104,24996898556928,118286594002944]