Strings and Languages
A string is simply the concatenation of several letters in an
alphabet. Typically, we will define an alphabet as a set
Σ
. So,
for example, we could have
Σ
= {a,b}.
Then, a string over this alphabet
Σ
, would be any “word”
formed with only the letters or characters a and b. There is no
limit on the length of a string. It must simply be a non negative
integer.
This means there is a string of length of length 0. This is known
as the empty string. The empty string is typically denoted by
λ
.
In particular, if you concatenate the empty string with any
other string, you get back that string. It seems silly to have an
empty string, but it will help out in certain situations. Make
sure you do not confuse the empty string with the empty set.
Also, recognize that
λ
can never be a letter of an alphabet.
In the book, they define nonempty strings in the following
manner :
Σ
n
is a string of length n over the alphabet
Σ
, and is defined as
below:
1)
Σ
1
=
Σ
.
2)
Σ
n+1
= {xy  x
∈Σ
, y
∈Σ
n
}
And any string is simply a subset of
Σ
n
, where n is a positive
integer.
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 Fall '09
 Formal language, Formal languages, Regular expression, Regular language

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