n/3 analog of Keith numbers.

Terms

a(0) =45a(1) =609a(2) =1218a(3) =1827a(4) =3213a(5) =21309a(6) =28206a(7) =29319a(8) =31917a(9) =39333a(10) =47337a(11) =78666a(12) =102090a(13) =117999a(14) =204180a(15) =406437a(16) =302867592a(17) =4507146801a(18) =5440407522