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";