When did Joseph Liouville begin and end the time as an employer at Collège de France?
SELECT ?value1 ?value2 WHERE { wd:Q214549 p:P108 ?s . ?s ps:P108 wd:Q202660 . ?s pq:P582 ?value1 . ?s pq:P580 ?value2 }
joseph liouville: Q214549 collège de france: Q202660