How to transform pg_7.4 function 'to_number' to 8.4 equivalent?

Hi all,

after upgrading from pg_7.4 to pg_8.3  when I run this command:

psql finanza -c "SELECT titolo, var, pr_chius, pr_aper, \
diff_aper_ch*(-1) as \"diff. ass.\", \
to_number(diff_aper_ch*(-1)/pr_aper*100::float(8),'9999.000000') as \"sc. %\" \
from ult_qq having pr_chius>pr_aper and pr_aper>0 \
order by titolo;"   | tee -a /home/ennio/.tmp/aaa

I get:

ERROR: function to_number(double precision, unknown) does not exist ...

Of course it worked very well under 7.4 ...

Can somebody suggest how to correct this?

Regards,
    Ennio

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