a(-1)=H_4(2,-1)=0;
a(0)=H_4(2,0)=1;
a(1)=H_4(2,1)=2;
a(2)=H_4(2,2)=2^2=4;
a(3)=H_4(2,3)=2^2^2=16;
a(4)=H_4(2,4)=2^2^2^2=65536;
a(3)=16组不超过3的秩为:
01: {}
02: {{}}
03: {{}, {{}}}
04: {{{}}}
05: {{}, {{}}, {{}, {{}}}}
06: {{}, {{}}, {{}, {{}}}, {{{}}}}
07: {{}, {{}}, {{{}}}}
08: {{}, {{}, {{}}}}
09; {{}, {{}, {{}}}, {{{}}}}
10: {{}, {{{}}}}
11: {{{}}, {{}, {{}}}}
12: {{{}}, {{}, {{}}}, {{{}}}}
13: {{{}}, {{{}}}}
14: {{{}, {{}}}}
15: {{{}, {{}}}, {{{}}}}
16: {{{{}}}}
(结束)