Perhaps you should try some spelling variations:
22international trade » international trade, vinternational trade, international grade
mathematische models » mathematische modell, mathematischen modells, mathematisches modells
auf mathematische » 1 mathematische, 2 mathematische, 3 mathematische
2f congresses » _ congresses, 6 congresses, 1 congresses

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• Adding a wildcard symbol may retrieve word variants: %22international trade %auf mathematische models %2f congresses%22*.