FreeMat is packaged for Ubuntu 18.04 LTS, it has necessary desktop-file in the package.
To correctly install the application on Ubuntu 18.04 LTS use the following commands:
sudo add-apt-repository universe
sudo apt-get update
sudo apt-get install freemat
For Ubuntu 20.04 LTS (and really newer) there is a problem - this version does not ship Qt4 in the repositories. So we need to use special third-party PPA to get it:
sudo add-apt-repository ppa:rock-core/qt4
sudo apt-get update
cd ~/Downloads
wget http://archive.ubuntu.com/ubuntu/pool/main/libf/libffi/libffi6_3.2.1-8_amd64.deb
wget http://archive.ubuntu.com/ubuntu/pool/universe/f/freemat/freemat_4.2+dfsg1-6_amd64.deb
wget http://archive.ubuntu.com/ubuntu/pool/universe/f/freemat/freemat-data_4.2+dfsg1-6_all.deb
wget http://archive.ubuntu.com/ubuntu/pool/universe/f/freemat/freemat-help_4.2+dfsg1-6_all.deb
sudo apt-get install ./libffi6_3.2.1-8_amd64.deb ./freemat_4.2+dfsg1-6_amd64.deb ./freemat-data_4.2+dfsg1-6_all.deb ./freemat-help_4.2+dfsg1-6_all.deb
The above method may be adapted for Ubuntu 21.04 using commands below:
sudo add-apt-repository ppa:rock-core/qt4
sudo sed -i "s/hirsute/focal/g" /etc/apt/sources.list.d/rock-core-ubuntu-qt4-hirsute.list
sudo apt-get update
cd ~/Downloads
wget http://archive.ubuntu.com/ubuntu/pool/main/libf/libffi/libffi6_3.2.1-8_amd64.deb
wget http://archive.ubuntu.com/ubuntu/pool/universe/f/freemat/freemat_4.2+dfsg1-6_amd64.deb
wget http://archive.ubuntu.com/ubuntu/pool/universe/f/freemat/freemat-data_4.2+dfsg1-6_all.deb
wget http://archive.ubuntu.com/ubuntu/pool/universe/f/freemat/freemat-help_4.2+dfsg1-6_all.deb
sudo apt-get install ./libffi6_3.2.1-8_amd64.deb ./freemat_4.2+dfsg1-6_amd64.deb ./freemat-data_4.2+dfsg1-6_all.deb ./freemat-help_4.2+dfsg1-6_all.deb