Earlier today Barry Cipra sent the following note as a comment to the post jsMath:
Website weirdness: The nabla symbol has suddenly disappeared from the equation on my browser (Safari, on a MacBook). It was there the last time I checked (a day or two ago), but now… gone!
With the generous help of Davide Cervone, author of jsMath, I’ve learned that the disappearing-nabla trick is not magic, and it doesn’t really have anything to do with jsMath. Barry is right that the character was present last week and that it was gone this morning. What happened in between is that bit-player.org (and my other web sites, industrial-landscape.com and grouptheoryinthebedroom.com) were transferred to new server hardware by the company that hosts my little Internet empire. It appears that several hundred files were left behind. Thanks, Bluehost.
Among the missing files, it seems, were some holding the PHP code that runs this blog. As a result, I couldn’t log in or even post an explanation of what had gone wrong. The only remedy was to restore a backup from before the migration. This has now been done, and I can confirm that the nabla is back. I need to check to see what other damage might remain. I’d be grateful for reports of any further weirdness: email@example.com