SQL & PL/SQL

Hi!

Somebody knows a nice trick for getting back a date from a given week number?

For example: i give a week number as parameter to a report with to_char ( sysdate, 'IWYYYY' );
Now, how can i get back the date from that string?

select to_date ( to_char ( sysdate, 'iwyyyy' ), 'iwyyyy' ) from dual;

throws: ORA-01820: format code cannot appear in date input format

Thanks in advance!

Regards