a(n) = (10^n - 1)*(10^(2*n-1) - 1)/81.A053655a(n) = (10^n - 1)*(10^(2*n-1) - 1)/81.Terms a(0) =1a(1) =1221a(2) =1233321a(3) =1234444321External referencesoeis: A053655