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Let's say an algorithm is published and then afterwards it is implemented under GPLv3. Although the algorithm is mostly abstract, it does refer to specifications from a de facto standard in order to do its job.

Briefly, the stages are:

  1. Algorithm is published
  2. Implemented in GPLv3

Does implementing the algorithm under GPLv3 restrict its, i.e. the algorithm, re-implementation under permissive licenses? Are the latter derivative work if they are purely based on the original algo and not on whatever mods were added in step 2?

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You can implement the algorithm multiple times with different license schemes, as long as you don't copy the code from another implementation.

If you copy code then you need to adhere to the requirements imposed by the existing license, and in the example you mention above you would not be able to use code from a GPL-3 project in an implementation licensed under a permissive license.

You need to check under which license conditions the algorithm itself is published. 'De Facto Standard' does not automatically imply that it can be used for free.

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  • I think my use of language here has been careless. What I meant by mod is the informal word for modification in general and not specific to Apache.
    – candor
    Oct 27 at 8:31
  • @candor I just modified my answer to make it more generic and remove 'Apache' which anyhow was only listed as an example for a permissive license. Oct 27 at 9:38

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