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

Try widening your search to All Fields.

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• Adding a wildcard symbol may retrieve word variants: %22international trade %oemof mathematische models %_ congresses%22*.