Bijective Function Domains on Words in the English Language

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.

Example

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.

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