a(n) = 4*a(n-1) - 2*a(n-2) for n > 1; a(0) = 3, a(1) = 14.

Terms

a(0) =3a(1) =14a(2) =50a(3) =172a(4) =588a(5) =2008a(6) =6856a(7) =23408a(8) =79920a(9) =272864a(10) =931616a(11) =3180736a(12) =10859712a(13) =37077376a(14) =126590080a(15) =432205568a(16) =1475642112a(17) =5038157312a(18) =17201345024a(19) =58729065472a(20) =200513571840a(21) =684596156416