There is Linked Open Numbers project. I don't understand how I can use it. It can be used with sparql query, I think. But I can't do it. I have found http://km.aifb.kit.edu/projects/numbers
Do you know that Linked Open Numbers was an April fools' day joke?
Note the publishing date on the project website. The paper accompanying the project is firmly tongue-in-cheek, and was published in RAFT, a joke journal.
At the time, many Semantic Web research groups were republishing existing data sets in Linked Data format, sometimes without much consideration for the actual usefulness of doing so. Linked Open Numbers pokes fun at that fashion. Linked Open Numbers is a very earnest effort at doing something that is entirely useless. Except, maybe it isn't useless after all? Maybe we just haven't opened our minds to the possibilities? Who knows! It's just like the entire Semantic Web effort!
With that out of the way, you asked how you can use the site, so I will explain.
First, there is no SPARQL endpoint, and no downloadable RDF dump either. There are an infinite number of numbers, so the storage required would be infinite!
The only way to use the site is to look up the information it has for any particular number. For example, for the number 42, we have:
So you can write code that downloads the RDF/XML data about a particular number, and accesses the various properties of the number, such as its spelling in different languages, its Roman numeral, and its prime factors. Most RDF libraries will make this job very easy.
And if you publish a dataset in RDF format and/or as linked data, and users of the dataset could potentially benefit from this information about numbers in the dataset, then you can include RDF links from numbers in your data to Linked Open Number's linked data URIs.
Is any of this useful? I don't know!