Consider two versions of What is the modulus of continuity of this function that is continuous, increasing and bounded but nowhere locally Hölder?. Notice that, in the first version, the linked question title does not expand; whereas, in the second version, it does:

Question title does not auto-expand

Question title auto-expands

My guess, which seems to be borne out by the experiment shown in these two screenshots, is that it is the ‘ö’ of ‘hölder’ in the URL that is the trouble.

See also my earlier question When are question titles auto-expanded? and @JohnOmielan's comment, linking <s>links</s> are autocompleted on preview but not rendered when posted.

  • 2
    $\begingroup$ You'll need to replace the Unicode ö with its HTML encoding for the auto-expansion to occur (I think this is by design). If you use the 'Hyperlink' tool (Ctrl+L) from the editing toolbar, then it will correctly encode such characters for you. Some related posts on Meta SE: Diamonds (♦) break the automatic hyperlink parser and Allow URL with diacritics. (Also, note that such a URL might not be auto-expanded in the preview, and yet be correctly auto-expanded in the final post.) $\endgroup$ Dec 8, 2023 at 20:44
  • $\begingroup$ @TheAmplitwist, re, thanks! It was not correctly expanded in rendered post; see mathoverflow.net/revisions/460029/2. I think that the issue is slightly different from that in Diamonds (♦) break the automatic hyperlink parser, because links are parsed correctly, just not expanded. I guess that, if an SE rep says it's by design, then that's an answer …. $\endgroup$
    – LSpice
    Dec 8, 2023 at 20:46
  • $\begingroup$ But in revision 2, the URL uses ö, doesn't it? Yes, the issue in that Meta SE post was much worse, but it was also caused by the special characters in the URL, as per Martijn Pieters's answer there. $\endgroup$ Dec 8, 2023 at 21:24
  • $\begingroup$ Ah, I think my parenthetical remark was unclear. I meant that after replacing the Unicode characters by their HTML encodings, the URL should be correctly auto-expanded in the final post, even if it does not appear auto-expanded in the preview. $\endgroup$ Dec 8, 2023 at 21:26


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