Change Algorithm Numbering

[Pascal Francq (Mailing lists)]

Hi,
I try to change the change the numbering schema of algorithms in the
form 'A1', 'A2', etc.
So I add the following command in the preamble of the document:
        \renewcommand{\thealgorithm}{A\arabic{algorithm}}
But LyX says it "thealgorithm" is undefined.
Any one an idea?
Thanks in advance.
-- 
Pascal Francq