Perhaps you should try some spelling variations:
"petri / iberieli / ca. 411-ca. 490" » "petri / iberici / ca. 411-ca. 490"
"petri / iberien / ca. 411-ca. 490" » "petri / siberien / ca. 411-ca. 490", "petri / iberian / ca. 411-ca. 490", "petri / iberin / ca. 411-ca. 490"

Try widening your search to All Fields.

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• The words AND, OR and NOT may confuse the search; try adding quotes: "((\"petri / iberieli / ca. 411-ca. 490\") OR (\"petri / iberien / ca. 411-ca. 490\"))".
• Removing quotes may allow a broader search: (( petri / iberieli / ca. 411-ca. 490 ) OR ( petri / iberien / ca. 411-ca. 490 )).
• Adding a wildcard symbol may retrieve word variants: (("petri / iberieli / ca. 411-ca. 490") OR ("petri / iberien / ca. 411-ca. 490"))*.