Hi there,

I got a question about quasiconcavity:

When we use the special type of bordered hessian to check a function's quasiconcavity, is it necessary that the 2nd leading principal minor is "strictly negative" and the 3rd leading principal minor is "strictly positive" as well? Actually, in the Exercise A2.18(b), the bordered hessian of f(x1, x2) has the 3rd leading principal minor "equal" to zero when x1 and x2 are zero. According to the rules given in the question, this function fails to be a quasiconcave. Am I right?

Thank you very much!!