YEARWEEK() in Oracle10g?

Question

In MySql I can use YEARWEEK() to receive the week and the related year of this week in one string. (E.g. SELECT YEARWEEK('1987-01-01'); which leads to "198653").

Is there anything like that in Oracle10g?

I only know about the TO_CHAR function. But if I use TO_CHAR(sysdate, 'YYYYIW'); I receive 198753 and not 198653. So, how I am able to calculate this correctly?