G.f.: (1+4*x+10*x^2+4*x^3+x^4)/(1-2*x-2*x^2-2*x^3+x^4).

Terms

a(0) =1a(1) =6a(2) =24a(3) =66a(4) =192a(5) =558a(6) =1608a(7) =4650a(8) =13440a(9) =38838a(10) =112248a(11) =324402a(12) =937536a(13) =2709534a(14) =7830696a(15) =22631130a(16) =65405184a(17) =189024486a(18) =546290904a(19) =1578810018a(20) =4562845632a(21) =13186868622a(22) =38110757640a(23) =110142133770a(24) =318316674432a(25) =919952263062