a(n) = binomial(9*n, 3*n) / (6*n + 1).A235535a(n) = binomial(9*n, 3*n) / (6*n + 1).Terms a(0) =1a(1) =12a(2) =1428a(3) =246675a(4) =50067108a(5) =11124755664External referencesoeis: A235535