Old Links to PlanetMath have Broken

Question

I recently fixed a broken link to a PlanetMath page, by changing http://planetmath.org/encyclopedia/RotationalInvarianceOfCrossProduct.html to http://planetmath.org/RotationalInvarianceOfCrossProduct.

Some links also have .html at the end, without /encyclopedia/ see e.g. http://planetmath.org/FullyIndecomposableMatrix.html from this Answer.

I also found some other links that I cannot understand, but now link to the homepage. These links start with planetmath.org/?op and now redirect to the homepage. I'm not sure what they were supposed to link to, however. I found this by performing a search for PlanetMath on MO. An example post there is this one.

There are also posts starting with planetmath.org/sites (as pointed out by Martin/hardmath). Some of these even link to PDF files that are no longer there.

As mentioned by Glorfindel, some posts are not fixed by the above procedure. I would guess that some pages have been renamed or deleted. It might be worth pointing out that PlanetMath has an index page which we can use to check before making the change.

Here are the posts that we can be sure have broken links:

1. (From Martin's comment) Searching$^{1}$ with url:"*planetmath.org/encyclopedia*" gives 178 hits.

2. Searching$^2$ with url:"*planetmath.org/?op*" gives a further 17 hits.

3. Trying to find posts with .html at the end: url:"*planetmath.org/*html" gives 248 results. If the wildcard * works in the middle of the search input, this should strictly contain the results of point 1.

• Are there other possible formats of PlanetMath links that are now broken?

• How shall we go about fixing the links?

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^{1. Note: searching for "PlanetMath" only gives 146 hits, but "url:"*planetmath.org/*"" gives 953 hits. So searching just for the website name does not search in the embedded links. Also note that wildcards * are supported in SE's built-in search.}

Answer 1

Because the ideal tool for a job like this, the mass-replacement tool, which can repair posts without bumping them, doesn't work in this case, I have fired up a script (a derivation of the Broken Image Repairer) to repair these links on half a dozen websites across the network, including Mathematics Stack Exchange. The script runs once every three days, bumping three questions at a time, in order not to flood the homepage with trivial edits.

I just did a dry run of the script and here is a list of URLs it won't be able to replace; these will need manual action. Here is another list with replacements it will make (scroll to the right to see the actual replacement links). You'll notice that it will update other links, e.g. HTTP to HTTPS. (I see some replacements are not necessary, e.g. for books.google.com. It's good to see the results upfront...)

I'm open for objections or suggestions. Following Martin Sleziak's comment, I can have the script try to repair planetmath.org/sites links as well, e.g. https://planetmath.org/sites/default/files/texpdf/40203.pdf in this question has a Wayback Machine copy.

Answer 2

I researched the twenty-four posts that Martin's planetmath.org/sites query finds. One post has three such links (the rest have one apiece), and one linked URL is shared by two posts, but two dozen is a pretty accurate estimate of the broken links.

UPDATE: All the posts in that batch are remedied. As Calvin Khor points out, Martin's query still finds one match, but this is because the Wayback archive copy of the PDF was retained in parallel with the newer equivalent HTML topic page.

In the meantime Martin Sleziak brought to my attention a different family of broken PlanetMath links, at MathOverflow as well as here. Before MathJax was a breakout success, there was a similiar package jsMath. These have been broken for a long time, and there is some variability to their syntax. Martin's search query is:

url:"*planetmath.org/?op=getobj*"

Comments were left on these posts; all but three are fixed.

Answer 3

I'm done replacing the links in the planetmath-failures list provided by Glorfindel.

Methodology: It seems the errors were because the URL were changed to match the title. Given this, all I had to do was find the posts on the Wayback Machine and find the title in the index. There were also some other non-Planetmath links; all but two (#1, #2) were fixed; some comments were left on the exceptions.

Notably I found some links that were not in the list. These seem to be links that were 'bare', i.e. without any markdown or HTML formatting to include them as hyperlinks.

This table details exactly what I did.