G.f.: Sum_{n=-oo..+oo} ( x^n / (1 - x^(2*n)*(1+x)^(n+1)) )^2.

Terms

a(0) =1a(1) =0a(2) =0a(3) =0a(4) =2a(5) =0a(6) =6a(7) =6a(8) =11a(9) =16a(10) =38a(11) =58a(12) =103a(13) =152a(14) =267a(15) =486a(16) =861a(17) =1392a(18) =2276a(19) =3812a(20) =6562a(21) =11224a(22) =18906a(23) =31446a(24) =52392a(25) =87776a(26) =147423a(27) =246698a(28) =411358a(29) =685988