[Couenne] Unknown operator error

[Pietro Belotti]

> Hmmm I'm not sure with it.
> If I replace
>  sum{j in J} (y[i,j]+eps)*ur[i]*...)^r[i]
> with
>  sum{j in J} y[i,j]*ur[i]*...)^r[i]
>
> I get several warnings suggesting to enable the "halt_on_ampl_error"
> option to see more details (nevertheless the optimal solution is
> reached).
> If I enable that option, couenne stops with the following error (in
> this case r[i] == 0.355):
>
> couenne: halt_on_ampl_error=yes
>
> ANALYSIS TEST: NLP0012I
>              Num      Status      Obj             It       time
>          Location
> NLP0014I             1         OPT 0.917700545.5g      785 2.121677g
> Error evaluating objective function: can't evaluate pow'(0,0.355).
> exit code 1
> <BREAK>

This error is returned by Ipopt while trying to compute a feasible 
solution of the continuous relaxation. However, the call to Ipopt does not 
have to terminate successfully for Couenne to work, and you can set 
halt_on_ampl_error back to no and ignore the warnings. Unfortunately AMPL 
may print many of them, but they should not prevent Couenne from finding 
an optimal solution.

Pietro