a(2)=1到a(4)=15图形项链的不等价代表:
{{12}} {{12}{13}} {{12}{34}}
{{12}{13}{23}} {{13}{24}}
{{12}{13}{14}}
{{12}{13}{24}}
{{12}{13}{34}}
{{12}{14}{23}}
{{12}{24}{34}}
{{12}{13}{14}{23}}
{{12}{13}{14}{24}}
{{12}{13}{14}{34}}
{{12}{13}{24}{34}}
{{12}{14}{23}{34}}
{{12}{13}{14}{23}{24}}
{{12}{13}{14}{23}{34}}
{{12}{13}{14}{23}{24}{34}}