Database: scholar
Question: When did Peter Mertens and Dina Barbian collaborate ?
SQL:
SELECT DISTINCT t3.paperid FROM writes AS t3 JOIN author AS t2 ON t3.authorid  =  t2.authorid JOIN writes AS t4 ON t4.paperid  =  t3.paperid JOIN author AS t1 ON t4.authorid  =  t1.authorid WHERE t2.authorname  =  "Peter Mertens" AND t1.authorname  =  "Dina Barbian";