Sqrt(0) problem

[Marqoos]

Hi. I'm begginer with AMPL. My problem is rather simple, number of
constraines and variables is about 10-12. The problem is, that I must
use sqrt function in constraines, but in first iteration it is sqrt(0).
Ampl solver can not obtaine derivitaive in this point (I guess so), I
get the communicate about failure error, and get no results. Is there
any possibility of avoiding this problem?
Regards.

Marqoos

[trah...@gmail.com]

I tried AMPL to find sqrt(0) and it works fine.
param z;
let z:=sqrt(0);
display z;

AMPL displays z = 0.

[Robert Fourer]

> From: am...@googlegroups.com [mailto:am...@googlegroups.com] On Behalf Of
> Marqoos
> Sent: Wednesday, November 16, 2005 10:56 AM
> To: AMPL Modeling Language
> Subject: [AMPL] Sqrt(0) problem

Try setting the initial values of your variables to something other than zero.
Initial values are discussed in section 18.4 of the AMPL book (2nd edition).

Bob Fourer
4...@ampl.com

[Marqoos]

There is no need to evalueate derivate from parameters so for
parameters it always warks fine. Try something like this.

var y;
var x;

maximize hej:
y;

subject to up_and_down_and_left:
y <= sqrt(abs(x));

subject to right:
x <= 4;

solve;

display y;

[Marqoos]

Thak you Robert I'll try, but AMPL book is in our university library
but it took ten days to get it (recall procedure). So usuwally I can
find some solutions faster. The efect is that I haven"t red it until
know. Maybe this time I will start the recall procedure.

[Marqoos]

I thin i found much better way of this regularisation. I just added
"small number" to expresion under square root. 10e-8 fo example. And
now everything works fine, even if expresion tends to zero.