Back Epsilon is missing in the Elements panel, under "Others" category. The Help is correct in mentioning it: "A <emph>back epsilon</emph> can be inserted by typing <emph>backepsilon</emph> in the <emph>Commands</emph> window." Please add backepsilon to the elements panel.
I submitted a patch for review here: https://gerrit.libreoffice.org/c/core/+/110880
Julien Nabet committed a patch related to this issue. It has been pushed to "master": https://git.libreoffice.org/core/commit/1fa83df607b9721c8f12125942de1c586a624bd0 tdf#140386: Math: Add 'backepsilon' symbol to elements panel It will be available in 7.2.0. The patch should be included in the daily builds available at https://dev-builds.libreoffice.org/daily/ in the next 24-48 hours. More information about daily builds can be found at: https://wiki.documentfoundation.org/Testing_Daily_Builds Affected users are encouraged to test the fix and report feedback.
Xisco Fauli committed a patch related to this issue. It has been pushed to "master": https://git.libreoffice.org/core/commit/9b52e7ab543406783a1535d03856a8b13b554a70 tdf#140386: math: Add UItest It will be available in 7.4.0. The patch should be included in the daily builds available at https://dev-builds.libreoffice.org/daily/ in the next 24-48 hours. More information about daily builds can be found at: https://wiki.documentfoundation.org/Testing_Daily_Builds Affected users are encouraged to test the fix and report feedback.