Expansion of Sum_{n>=0} Product_{k=0..n} (x^k*(1+x)^(n-k) + x^(n-k)*(1+x)^k).

Terms

a(0) =3a(1) =6a(2) =15a(3) =36a(4) =98a(5) =258a(6) =677a(7) =1830a(8) =5006a(9) =13340a(10) =35215a(11) =95702a(12) =264851a(13) =717760a(14) =1894473a(15) =5031846a(16) =13788409a(17) =38375030a(18) =105005017a(19) =279236168a(20) =734728565a(21) =1967715202a(22) =5416631023a(23) =15061949148a(24) =41271428388a(25) =110250824636a(26) =289840310574a(27) =766277436248