a(n) = (28^n - 1)/27.A218731a(n) = (28^n - 1)/27.Terms a(0) =0a(1) =1a(2) =29a(3) =813a(4) =22765a(5) =637421a(6) =17847789a(7) =499738093a(8) =13992666605a(9) =391794664941External referencesoeis: A218731