a(n) = Stirling2(2*n+1, n).A247238a(n) = Stirling2(2*n+1, n).Terms a(0) =1a(1) =15a(2) =301a(3) =7770a(4) =246730a(5) =9321312a(6) =408741333a(7) =20415995028External referencesoeis: A247238