Perhaps you should try some spelling variations:
22international trade » international trade, vinternational trade, international grade
mathematische models » mathematische modell, mathematischen modells, mathematisches modells
oemof mathematische » oemof mathematical, auf mathematische, f mathematische
2f congresses » _ congresses, 6 congresses, 16 congresses
1 congresses » 16 congresses, _ congresses, 6 congresses

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• The words AND, OR and NOT may confuse the search; try adding quotes: "%22international trade %oemof mathematische models %((2f congresses) OR (1 congresses))%22".
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