a(n) = (27^n - 1)/26.A218730a(n) = (27^n - 1)/26.Terms a(0) =0a(1) =1a(2) =28a(3) =757a(4) =20440a(5) =551881a(6) =14900788a(7) =402321277a(8) =10862674480a(9) =293292210961External referencesoeis: A218730