If you were to apply a bijective function to each letter in each word of a language (e.g. English), how many pre-existing words would you obtain in the resulting image?
Since that's a pretty convoluted way of explaining things, let's try a more concrete example.
We'll take the well-known
rot13 substitution cipher (a simple example of a
bijection between the set of letters in the English alphabet and itself), and
apply it to every letter in a chosen word. For most words, the result will be
non-sensical gibberish. There does exist, however, a subset of valid English
words that map into other valid English words.
rot13('sync') = 'flap'
How many of these words exist? To answer this, I wrote a small Python script that loads up the words in my system dictionary into a Redis set. Another set of the rot13'ed words is then stored, and the set intersection of the original and transformed words is calculated:
After a simple cardinality check, we have our answer: 256 words1.
This result does not discard any single letter words (e.g. "a"), which will always trivially map into another letter when using rot13.