The following questions are rather theoretical/academical in nature.
According to Wikipedia
a system of data-manipulation rules (such as a computer's instruction set, a programming language, or a cellular automaton) is said to be Turing-complete or computationally universal if it can be used to simulate any Turing machine.
It is said that TeX is Turing-complete.
I think about the implications of these statements.
There are several TeX-engines—e.g., Knuth's TeX, ε-TeX, pdfTeX, XeTeX, LuaTeX, uTeX, upTeX.
Ouestion 1:
Are these TeX-engines Turing-complete?
Ouestion 2:
Are these TeX-engines Turing-machines?
If so:
Question 3:
Is it possible to simulate, e.g., a LuaTeX engine (with unicode as internal character encoding scheme) on an 8-bit Knuth TeX engine (with ASCII as internal character encoding scheme)?
"Being a Turing-machine and not something whereof a Turing-machine is a strict subset" and "Being Turing-complete" are different properties. If TeX-engines are Turing-complete without being just Turing-machines themselves, you cannot conclude that different TeX-engines can simulate one another.
Question 4:
Is Turing-machinery a strict subset of these TeX-engines?
Do TeX-engines have components which do not belong to the concept of a Turing-machine?
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