a(0)=1:{}。
a(1)=2:{aa},{a,a}。
a(2)=49:{aaab}、{aaba}、{aabb}、{abaa}、{abab}、{abb}、{baa}、{baab}、{baba}、{babb}、{bba}、{bbab}、{bbba}、{a,aab}、{a,aba}、{a,abb}、{a,baa}、{a,bab}、{a,bba}、{a,bbb}、{aa,ab}、{aa,ba}、{aa,bb}、{aaa,b},{aab,b},{ab,ab},{ab,ba},{ab,bb},{aba,b},{abb,b},{b,baa},{b,bab},{b,bba},{ba,ba},{ba,bb},{a,a,ab},{a,a,ba},{a,aa,b},{a,ab,b},{a、b、ba},}a,b,bb}、{a,b、b}、}ab、b、b{,b{、b{b、b、ba、{a、a、a和b}。