datepicker is not a function

datepicker is not a function

Hi all,
i'm using jQuery UI Datapicker, and i have a problem
this is mi code:
$(document).ready(function()
{
    $("#rangeDatepicker").datepicker(
        {
            numberOfMonths: [3, 4],
            onSelect:function(date, inst){
                alert($(inst).datepicker('getDate'));
//alert($(this).datepicker
('getDate'));
            }
        }
    );
$("#rangeDatepicker").datepicker("setDate",new Date(2009, 1-1, 1));
});
When the page is ready the datepicker is displayed correctly, and date
is setted at 2009-01-01: all works.
But cliccking on a day, i recive an error from firebug: "$
(inst).datepicker is not a function"
(or "$(this).datepicker is not a function" if i use this).
And i don't understand WHY??
What i wanna do is to modify option of datapicker on the event
onSelect. But i can't because it doesn't find the
function .datapicker.
Somebody can help me, please ??
Thanks
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