The Ubuntu Linux version of DirectMath does not offer a link to Mathematica, but will work with Maxima. Beyond the absence of the Mathematica link, the Linux version of DirectMath retains all of the features of the standard version.
DirectMath software requires the purchase of an unlocking code for $35 to unlock the save command. Beyond the locked save command, the version available here is fully functional.
Purchasing an unlocking code gives a single user the right to run DirectMath software on multiple computers and multiple operating systems. The unlocking code will unlock the current version and all future upgrades of the DirectMath software.
Version 3.2 for Ubuntu 14.04 64 bit
Version 3.1 for Ubuntu 16.04 64 bit
Double-clicking the packages after downloading will launch the Ubuntu Software Center application, which will give you the option to install the software.
Alternatively, you can install the software by opening a terminal window, changing to the directory where you downloaded the package, and typing
sudo dpkg -i directmath_3.2.0_amd64.deb
The packages linked to above are all signed. If you would like to verify the signature on these packages to confirm that they do originate from the author of DirectMath, please follow the steps below.
To check the signature on any of the DirectMath packages, start by fetching the public key for the gpg key used to sign the DirectMath package from the Ubuntu key server by going to keyserver.ubuntu.com and entering the search term email@example.com. Paste the public key block into a text editor and save the file as directmath.key.
To verify the package signature on the DirectMath package type the commands
gpg --import directmath.key sudo apt-get install dpkg-sig dpkg-sig --verify directmath_3.2.0_amd64.deb
dpkg-sig should respond with a message starting with GOODSIG to indicate that the signature on the package is good.