a(n) = 3*A131090(n) - A131090(n+1).

Terms

a(0) =-1a(1) =3a(2) =-1a(3) =2a(4) =-1a(5) =5a(6) =6a(7) =17a(8) =27a(9) =58a(10) =111a(11) =229a(12) =454a(13) =913a(14) =1819a(15) =3642a(16) =7279a(17) =14565a(18) =29126a(19) =58257a(20) =116507a(21) =233018a(22) =466031a(23) =932069a(24) =1864134a(25) =3728273a(26) =7456539a(27) =14913082a(28) =29826159a(29) =59652325