Sentence:  This <WSD>is</WSD> an unsolved problem which probably has never been seriously investigated , although one frequently hears the comment that we have insufficient specialists of the kind who can compete with the Germans or Swiss , for example , in precision machinery and mathematics , or the Finns in geochemistry . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)