Not a DOI

The resolution service at https://doi.org resolves DOIs. You have attempted to resolve en/publishing-house/journals-and-series/fundamenta-mathematicae/all/4/0/92480/demonstration-elementaire-du-theoreme-sur-la-densite-des-ensembles, which is not a DOI.

en/publishing-house/journals-and-series/fundamenta-mathematicae/all/4/0/92480/demonstration-elementaire-du-theoreme-sur-la-densite-des-ensembles may be a Handle (DOIs are a special sort of Handle). If en/publishing-house/journals-and-series/fundamenta-mathematicae/all/4/0/92480/demonstration-elementaire-du-theoreme-sur-la-densite-des-ensembles is a valid Handle you should be able to resolve it at https://hdl.handle.net.