PlainTeX
PlainTeX is the first and simplest TeX Dialect. Original TeX compiler only supports 256 registers used by PlainTeX. LuaTeX supports 65536 however PlainTeX cannot use them.
main.tex:
Hello, \TeX! $$\sum_{n = 1}^\infty{1\over{n^2}} = {\pi^2\over6}$$ \bye
lx add -b luatex
lx shell --build
luatex main.tex