6*((10^n-1)/9)*(10^(n+1))+9*(10^n-1)/9.A2468806*((10^n-1)/9)*(10^(n+1))+9*(10^n-1)/9.Terms a(0) =0a(1) =609a(2) =66099a(3) =6660999a(4) =666609999a(5) =66666099999External referencesoeis: A246880