Start with a(1)=1; to get a(n) replace each i in a(n-1) with 12...i, then append n.

A004073

Start with a(1)=1; to get a(n) replace each i in a(n-1) with 12...i, then append n.

Terms

    a(0) =1a(1) =12a(2) =1123a(3) =11121234

External references