Number of vertices in Sierpiński triangle of order n.

Terms

a(0) =3a(1) =6a(2) =15a(3) =42a(4) =123a(5) =366a(6) =1095a(7) =3282a(8) =9843a(9) =29526a(10) =88575a(11) =265722a(12) =797163a(13) =2391486a(14) =7174455a(15) =21523362a(16) =64570083a(17) =193710246a(18) =581130735a(19) =1743392202a(20) =5230176603a(21) =15690529806a(22) =47071589415a(23) =141214768242a(24) =423644304723