Many posts on MO contain a link to imgur.com. Such links can be created outside the Stack Exchange network - if somebody simply creates a picture on imgur. But some of them might have been uploaded through the Stack Exchange interface. Currently such links will have the format i.stack.imgur.com rather than a link to i.imgur.com. But this was not always the case.

As far as I can tell, at some point there were two changes to the way images are uploaded in the Stack Exchange editor. Since August 2010, Stack Exchange can upload to an imgur account which is not subject to 6 months limit - making them more-or-less permanent.¹ This also led to changing the format of the links from i.imgur.com to i.stack.imgur.com. Stack Exchange updated the links from the old format to the new one network-wide.²

All of this happened before MathOverflow joined the Stack Exchange network. So the update by the Stack Exchange team did not influence MathOverflow. Was such an update perhaps done by the MO mods at the time? Or was it done by SE after MO joined the network?

It seems that there are still at least some links to i.imgur.com which could be fixed by i.stack.imgur.com.

Questions.

• What are the reasons for the broken i.imgur.com links which are supposed to be in the format i.stack.imgur.com. Is it likely that there are many such links? Or are these examples only some rare exceptions?
• Is it possible to find all links of this type? (I.e., the links where simply changing the URL to i.stack.imgur.com would fix the picture.)
• Was a clean-up of imgur links done on MathOverflow at some point? (Similar as the clean-up done on other sites - which were part of the network in 2010.)
• Can such links be fixed only manually? Or is it possible to do this automatically.

I suppose that there are only a very few posts of this type. Search returns only 111 posts with such links. SEDE returns 111 posts and 114 if we search in MarkDown.)

All posts of this kind (i.e., where adding stack into the URL helps) that I was able to find are listed below. As you can see, it is a rather short list.

At the same time, it is a bit unclear how the changes back in 2010 influenced MO - which was at the time outside of the network and on the older version of the Stack Exchange software. So it seemed reasonable to ask about this - perhaps there are some people who are aware of the details.

Examples

• I have recently edited the answer "What's your favorite equation, formula, identity or inequality?" (after the question was bumped to the frontpage). Here is a link to the original revision - I have only changed http://i.imgur.com/vCSQK.png to https://i.sstatic.net/vCSQK.png and the link seems to work now.
• Which platonic solids can form a topological torus? (current revision) in the current revision has a link to https://i.sstatic.net/9JaoX.gif - which does not work, but the link to https://i.sstatic.net/JfQZU.gif actually works. (I have noticed this post because it still has the deprecated tag geometry.)
• Symmetries of the Julia sets for $z^2+c$ (current revision) contains a link to http://i.imgur.com/xAZZb.png - while the link to https://i.sstatic.net/xAZZb.png works.
• The Hexagonal Property of Pascal's Triangle (current revision) with a link to http://i.imgur.com/pChFw.png - the link to https://i.sstatic.net/pChFw.png does work.
• Numerical instability using only Heun's method on a simple PDE. (the current revision) - contains a link to http://i.imgur.com/cZKaB.gif which does not work. The link https://i.sstatic.net/cZKaB.gif works - but this picture seems to be completely unrelated to the post.
• How to formulate such problem mathematicaly? (line continuation search) (the current revision) - contains these three links: http://i.imgur.com/76yeO.png, https://i.sstatic.net/dSTKe.png, https://i.sstatic.net/i8DAZ.gif. The corresponding links https://i.sstatic.net/76yeO.png, https://i.sstatic.net/VIOmp.png, https://i.sstatic.net/WSlIe.gif work. (This is another case of a question with the deprecated tag geometry.)

¹Stack Overflow blog: New Image Upload Support, Mathematics Meta: Permanent Picture Uploads and Broken imgur-links.

²Any details about stack.imgur.com? and Images manually uploaded to Imgur are not displaying?. These links were mentioned in a related conversation in chat.