4.21 how to install lyx on Linux

see http://wiki.lyx.org/LyX/LyXOnUbuntu

if not in the package manager PPA, then do

sudo add-apt-repository ppa:lyx-devel/daily (add the PPA) 
sudo apt-get update (update so that apt is aware of the new PPA) 
sudo apt-get install lyx2.0 (install from the PPA) 
lyx2.0 &