a(n) = floor(sqrt(3)*2^(n-1)).

Terms

a(0) =1a(1) =3a(2) =6a(3) =13a(4) =27a(5) =55a(6) =110a(7) =221a(8) =443a(9) =886a(10) =1773a(11) =3547a(12) =7094a(13) =14188a(14) =28377a(15) =56755a(16) =113511a(17) =227023a(18) =454046a(19) =908093a(20) =1816186a(21) =3632373a(22) =7264747a(23) =14529495a(24) =29058990a(25) =58117981a(26) =116235962a(27) =232471924a(28) =464943848a(29) =929887696