CREATE OR REPLACE FUNCTION f_week_to_date(a_week varchar2) RETURN CHAR IS

   v_first_date   char(10);

   v_date_of_week number(1);  

 BEGIN

   select to_char(to_date(substr(a_week, 1, 4) || '', 'yyyymmdd'), 'D')

     into v_date_of_week

     from dual;

   v_date_of_week := v_date_of_week - 1;

   if v_date_of_week <= 4 then

     select TO_CHAR(TO_DATE(SUBSTR(a_week, 1, 4) || '', 'yyyymmdd') +

                    SUBSTR(a_week, 5, 2) * 7 - 7 - v_date_of_week + 1,

                    'yyyy-mm-dd')

       into v_first_date

       from dual;

   else

     select TO_CHAR(TO_DATE(SUBSTR(a_week, 1, 4) || '', 'yyyymmdd') +

                    SUBSTR(a_week, 5, 2) * 7  - v_date_of_week + 1,

                    'yyyy-mm-dd')

       into v_first_date

       from dual;

   end if;  

   return v_first_date;  

 END;