The sequence c[n] defined in A126939.

Terms

a(0) =0a(1) =2a(2) =4a(3) =12a(4) =42a(5) =130a(6) =404a(7) =1282a(8) =4044a(9) =12726a(10) =40114a(11) =126450a(12) =398478a(13) =1255796a(14) =3957784a(15) =12473128a(16) =39309510a(17) =123885908a(18) =390432440a(19) =1230465890a(20) =3877871274a(21) =12221294900a(22) =38515986858a(23) =121384948432a(24) =382550391954