a(n) = 2*a(n-1) - a(n-2) + 2*a(n-3), with a(0) = 2, a(1) = 2.

Terms

a(0) =0a(1) =2a(2) =7a(3) =12a(4) =21a(5) =44a(6) =91a(7) =180a(8) =357a(9) =716a(10) =1435a(11) =2868a(12) =5733a(13) =11468a(14) =22939a(15) =45876a(16) =91749a(17) =183500a(18) =367003a(19) =734004a(20) =1468005a(21) =2936012a(22) =5872027a(23) =11744052a(24) =23488101a(25) =46976204a(26) =93952411a(27) =187904820a(28) =375809637a(29) =751619276