Hi Abhilash,
many thanks again for your help.
I have put a double ' instead of the comma
{INVENT.PART_NO} IN Split({?PART_NO}, '''')
and it works fine > I have the WHERE clause 
( except for the equality INVENT.PART_NO='' OR INVENT.PART_NO='' OR INVENT.PART_NO=',' which gives nothing)