a) List all the languages whose Kleene-star is finite.
b) Provide examples of two infinite languages X and Y , and a finite
language Z such that X ∩ Y is finite, and XZ is also finite. Explain
your answer.
(a) The empty language ∅ and the null language {ε} which
contains blank string are the only two languages with finite
Kleene-star or Kleene closure.
(b) Let X = {a, aa, aaa, aaaa, ....} = a*, Y = {a, b, bb, bbb,
bbbb, bbbbb, ....} = (a+b)* and Z is an empty language ∅.
Then, although X and Y are infinite languages, there intersection
{a} is finite as there is a finite number of strings common to X
and Y.
X is an infinite language but Z is a finite language. However, XZ
is the empty string ∅ which is also finite. This is because
concatenation of all strings of X with all strings of Z provides no
string as there is no string in Z.
Hope this helps.
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