> program was born out of the observation that many problems are commonly solved with a calculator, a pencil, and a sheet of paper

Few apps for Linux and Android: TreeSheets[0], µMath+[1], Xournal++[2].

[0] https://strlen.com/treesheets/

[1] https://f-droid.org/en/packages/com.mkulesh.micromath.plus/

[2] https://xournalpp.github.io/