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

Terms

a(0) =0a(1) =0a(2) =1a(3) =2a(4) =5a(5) =14a(6) =40a(7) =114a(8) =322a(9) =900a(10) =2490a(11) =6826a(12) =18564a(13) =50146a(14) =134690a(15) =360068a(16) =958826a(17) =2545082a(18) =6737860a(19) =17799570a(20) =46939378a(21) =123608388a(22) =325130970a(23) =854405962a(24) =2243591940a(25) =5887904194a(26) =15444237890a(27) =40495141124a(28) =106146043082a(29) =278161091930