Write 0, 1, ..., n in base 3 and add as if they were decimal numbers.

Terms

a(0) =0a(1) =1a(2) =3a(3) =13a(4) =24a(5) =36a(6) =56a(7) =77a(8) =99a(9) =199a(10) =300a(11) =402a(12) =512a(13) =623a(14) =735a(15) =855a(16) =976a(17) =1098a(18) =1298a(19) =1499a(20) =1701a(21) =1911a(22) =2122a(23) =2334a(24) =2554a(25) =2775a(26) =2997a(27) =3997a(28) =4998a(29) =6000