I noted in Slackware Diary that I had installed Free42 under Slackware. As I wrote in 2006 I fell in love with HP calculators and their RPN notation back in the 1970s, and still find the conventional algebraic notation rather awkward. I no longer carry a physical calulator, but I have Free42 installed on my phone and my tablet, and on my Windows systems. Now I also have it on Slackware. So I circled back to see if I could install in on Ubuntu.
The Ubuntu 20.04 repository includes
free42-nologo and I installed it with
sudo apt install free42-nologo. The installation ran OK. Like on Slackware no icon was created, but
/usr/bin/free42bin did appear. Since
/usr/bin/ is in $PATH, free42bin can be invoked from the command line. Unlike with Slackware and Windows, there is no free42dec. Invoking free42bin gave me an error message:
gmcdavid@Donegal:~$ free42bin &  213114  Done free42bin gmcdavid@Donegal:~$ Gtk-Message: 10:17:03.320: Failed to load module "canberra-gtk-module"
However, the calculator did appear on my desktop and it worked, at least for simple calculations. Stackoverflow suggested
sudo apt-get install libcanberra-gtk-module
and now free42bin works just like it does on Slackware. I do not understand what happened to free42dec.
Ubuntu 22.04 get it right.
sudo apt install free42-nologo installs both free42bin and free42dec, and both have icons to invoke them from the GUI.