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All previous supported versions are available from the downloads page. Any versions older than that aren't available. Also keep in mind that the numbering scheme changed starting in 2016. All versions prior to 2016 had a major and minor version (the last one was 7.11). All versions since have used the year and quarter for numbering scheme (e.g. 16.1 was the first with this format). All this to say that there was never a version 13.1
Thank you for the response but still I can not see this page. I get the fellow message:
An Error Has Occurred The page does not exist or you do not have permission to see it
Hmm, it's possible that only support contacts have the ability to download Appian. Did you complete all the steps here to self register (including registering with your company)? Also, can you go to the support page on Appian Community: https://community.appian.com/support/? If you have completed the registration and still don't have access, I'd suggest reaching out to your primary contact for support at your company.