Number of step shifted (decimated) sequences using a maximum of three different symbols.

Terms

a(0) =3a(1) =9a(2) =18a(3) =54a(4) =72a(5) =405a(6) =390a(7) =1944a(8) =3411a(9) =14985a(10) =17802a(11) =139968a(12) =133104a(13) =798525a(14) =1804518a(15) =5454378a(16) =8072532a(17) =64599849a(18) =64573626a(19) =437732424a(20) =872157294a(21) =3138159429a(22) =4279259574a(23) =35362084140a(24) =42364514403a(25) =211822562025a(26) =423646166250