- #1
Jagella
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An infinite number of different names might be listed in a telephone directory. For any conceivable name, a new and different name can be created by adding one letter.
Can any phone directory be created to include all conceivable names even if there are an infinite number of names?
It may be tempting to answer yes to this question by positing an infinite directory that can list an infinite number of names. Problem solved!
Strangely, even an infinite directory isn't big enough. Here's my reasoning: Consider the first three names listed in that directory.
These names can be composed of any number of letters, even an infinite number of letters. Even though there may be an infinite number of names, you can always make up a name that's not on the list! How? Make up that name by selected the letters on the diagonal and create a name using different letters than the letters on the diagonal. Say
Bef
Using different letters in this name than the names on the diagonal ensures that it has not yet been included in the directory. Bef cannot be the first name because the first letter differs from the first letter in the first name. Bef cannot be the second name because the second letter differs from the second letter in the second name. Finally, Bef cannot be the third name because the third letter differs from the third letter in the third name.
No matter how many names are in the directory, using the process of making up a name by replacing the letters on the diagonal with different letters and using those replaced letters in the new name guarantees that the new name will differ from all the names in the directory by at least one letter.
Am I right?
Jagella
Can any phone directory be created to include all conceivable names even if there are an infinite number of names?
It may be tempting to answer yes to this question by positing an infinite directory that can list an infinite number of names. Problem solved!
Strangely, even an infinite directory isn't big enough. Here's my reasoning: Consider the first three names listed in that directory.
- Aab...
- Abc...
- Aad...
These names can be composed of any number of letters, even an infinite number of letters. Even though there may be an infinite number of names, you can always make up a name that's not on the list! How? Make up that name by selected the letters on the diagonal and create a name using different letters than the letters on the diagonal. Say
Bef
Using different letters in this name than the names on the diagonal ensures that it has not yet been included in the directory. Bef cannot be the first name because the first letter differs from the first letter in the first name. Bef cannot be the second name because the second letter differs from the second letter in the second name. Finally, Bef cannot be the third name because the third letter differs from the third letter in the third name.
No matter how many names are in the directory, using the process of making up a name by replacing the letters on the diagonal with different letters and using those replaced letters in the new name guarantees that the new name will differ from all the names in the directory by at least one letter.
Am I right?
Jagella