a(n) = binomial(2*n,n) - binomial(2*n-2,n-1); or (3n-2)*C(n-1), where C = Catalan numbers (A000108).

A051924

a(n) = binomial(2*n,n) - binomial(2*n-2,n-1); or (3n-2)*C(n-1), where C = Catalan numbers (A000108).

Terms

    a(0) =1a(1) =4a(2) =14a(3) =50a(4) =182a(5) =672a(6) =2508a(7) =9438a(8) =35750a(9) =136136a(10) =520676a(11) =1998724a(12) =7696444a(13) =29716000a(14) =115000920a(15) =445962870a(16) =1732525830a(17) =6741529080a(18) =26270128500a(19) =102501265020a(20) =400411345620

External references