The first leading principal minor of HðQ; PÞ is simply jH11j, which is clearly negative for all values of Q and P. The second leading principal minor of HðQ; PÞ is equal to its determinant. Appendix A shows that the value of this determinant is positive for all Q and P as long as K > a. Therefore, the Hessian matrix is negative definite and consequently the profit function TPjðQ; PÞ is concave.