a(4)=1,a(6)=5,a(7)=14组分区:
{{13}{24}} {{135}{246}} {{13}{246}{57}}
{{13}{25}{46}} {{13}{257}{46}}
{{14}{25}{36}} {{135}{26}{47}}
{{14}{26}{35}} {{135}{27}{46}}
{{15}{24}{36}} {{136}{24}{57}}
{{136}{25}{47}}
{{14}{257}{36}}
{{14}{26}{357}}
{{146}{25}{37}}
{{146}{27}{35}}
{{15}{246}{37}}
{{15}{247}{36}}
{{16}{24}{357}}
{{16}{247}{35}}