In classical cryptography, a permutation cipher is a transposition cipher in which the key is a permutation.
To apply a cipher, a random permutation of size E is generated (the larger the value of E the more secure the cipher).
Considering the specific case of encrypting messages in English (i.e.
m = 26), there are a total of 286 non-trivial affine ciphers, not counting the 26 trivial Caesar ciphers.
Only those letters which occur in the English alphabet are affected; numbers, symbols, whitespace, and all other characters are left unchanged.
Because there are 26 letters in the English alphabet and 26 = 2 * 13, the ROT13 function is its own inverse: ROT13(ROT13(x)) = x for any basic Latin-alphabet text x An example plaintext to ciphertext using ROT13: A Polybius Square is a table that allows someone to translate letters into numbers.
These examples show what unaccommodatingness poems look like.
There is also a link below to the definition of unaccommodatingness and a page where you can discuss these types of poems.
This number comes from the fact there are 12 numbers that are coprime with 26 that are less than 26 (these are the possible values of a).
Each value of a can have 26 different addition shifts (the b value) ; therefore, there are 12*26 or 312 possible keys.
Poetry Soup is a great resource for examples of unaccommodatingness poems or a list of unaccommodatingness poetry.