a(n) = 5*F(n)^2 + 3*(-1)^n where F(n) are the Fibonacci numbers A000045.

Terms

a(0) =3a(1) =2a(2) =8a(3) =17a(4) =48a(5) =122a(6) =323a(7) =842a(8) =2208a(9) =5777a(10) =15128a(11) =39602a(12) =103683a(13) =271442a(14) =710648a(15) =1860497a(16) =4870848a(17) =12752042a(18) =33385283a(19) =87403802a(20) =228826128a(21) =599074577a(22) =1568397608a(23) =4106118242a(24) =10749957123a(25) =28143753122a(26) =73681302248a(27) =192900153617a(28) =505019158608