If you source from a wikipedia category, please specify from which page: there are occasionally more than a hundred WP pages on a given person. Besides, it is never necessary to source from Wikipedia: either the information is not sourced there, and then it has nothing to do on Wikidata, or it is correctly sourced there, and then why not simply provide the source on Wikidata also? The problem with Wikipedia categories is that most of the times they are sourced nowhere in the article. So you end up with circular references.
Regarding the assertion on Leonhard Euler you sourced from the English Wikipedia, it happens that it is currently sourced there by a dead link. I put a tag on it, and if no valid reference is provided, I will erase the category on Wikipedia, and the assertion imported from it on Wikidata.
Oh, one more thing: I am not loosing your time, you are loosing mine, by insisting in providing invalid references. ~~~~