a(n) = binomial((n+2)*(n+1),(n+1)*n).A136419a(n) = binomial((n+2)*(n+1),(n+1)*n).Terms a(0) =1a(1) =15a(2) =924a(3) =125970a(4) =30045015a(5) =11058116888External referencesoeis: A136419