Perhaps you should try some spelling variations:
22international trade » vinternational trade, international grade, international travel
mathematische models » mathematische modell, mathematischen modells, mathematisches modells
oemof mathematische » oemof mathematical, auf mathematische, f mathematische
2f congresses » _ congresses, 6 congresses, 1 congresses

Try widening your search to All Fields.

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• The words AND, OR and NOT may confuse the search; try adding quotes: "%((22international trade) OR (international trade)) %oemof mathematische models %2f congresses%22".
• Adding a wildcard symbol may retrieve word variants: %((22international trade) OR (international trade)) %oemof mathematische models %2f congresses%22*.