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This question probably belongs to some more general meta but since most heavily I use links on MO let me still ask it here.

Occasionally I link to materials which may disappear at some point - like an image found at some obscure place or an unpublished note on somebody's page at the current affiliation place which might change. While an image I can always just download and then upload, I wonder whether there is a quicker way to do it, and for other cases I don't know at all what to do.

I've heard of the terms "stable url", "persistent link", "permalink" but I don't know much about it.

Is there a way to use links on MO that would not expire?

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    $\begingroup$ Creating a stable link is a feature of the site you want to link to, not MO or any other site. If the original file (or the whole site) dies, the link dies. The only way to make a stable link to something stored in a suspicious place is to copy it to a more reliable storage and link there. $\endgroup$ – Joonas Ilmavirta Mar 22 '15 at 9:52
  • $\begingroup$ Think about linking like citing a paper. If you cite a paper from 1900, and there are no more copies of that paper around, the citation is a "dead link". It will survive if you copy it to some collection "All the Math Published in 1900" or whatever. One example is the Scott-Solovay paper about forcing with Boolean-valued models. It was never published, so all the references are "dead links". Luckily the content was passed on, and so this is not a big deal. $\endgroup$ – Asaf Karagila Mar 22 '15 at 14:14
  • $\begingroup$ Then what is a permalink? $\endgroup$ – მამუკა ჯიბლაძე Mar 22 '15 at 14:34
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    $\begingroup$ A permalink is a link about which the webmaster promises to make a reasonable effort to keep it working even if the site is reorganized in an unforseen way in the future. It does not differ form ordinary links in any technical or formal sense. $\endgroup$ – Emil Jeřábek Mar 22 '15 at 18:03
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    $\begingroup$ (Or in a forseen way, for that matter; many links are designed to have a changing content, whereas a permanent link is supposed to keep the same content.) $\endgroup$ – Emil Jeřábek Mar 22 '15 at 18:16
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The Internet Archive Wayback Machine allows you to save some web pages permanently as seen on at a given date and time. For example, I generated the following to save this page as it was before I answered:

http://web.archive.org/web/20150322201116/https://meta.mathoverflow.net/questions/2193/stable-url-for-linking

See the FAQ for the Wayback Machine for details on where, when and how you can use this service.

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