[Ipopt] A maximization problem returns a minimum

[Paul Smith]

Dear All,

I am running this simple model through AMPL and Ipopt 3.8.0:

--------------------------------------
var x >= 0;

let x := 0.5;

maximize F:	
   -16 * x * (x-1) * (2*x - 1)^2;

subject to R1:
   x <= 1;
--------------------------------------

Ipopt returns the following solution:

«EXIT: Optimal Solution Found.

Ipopt 3.8.0: Optimal Solution Found

suffix ipopt_zU_out OUT;
suffix ipopt_zL_out OUT;
ampl: display F, x;
F = 0
x = 0.5».

However the returned solution is a minimum and NOT a maximum. What am
I not understanding?

Thanks in advance,

Paul