a(n) = if n mod 2 = 0 then 8*F(n)-n otherwise 8*F(n)-4, where F() = Fibonacci numbers A000045.

Terms

a(0) =0a(1) =4a(2) =6a(3) =12a(4) =20a(5) =36a(6) =58a(7) =100a(8) =160a(9) =268a(10) =430a(11) =708a(12) =1140a(13) =1860a(14) =3002a(15) =4876a(16) =7880a(17) =12772a(18) =20654a(19) =33444a(20) =54100a(21) =87564a(22) =141666a(23) =229252a(24) =370920a(25) =600196a(26) =971118a(27) =1571340a(28) =2542460a(29) =4113828