I realize that the affine ciphers are not currently in use, but it still seems like a good idea to avoid propagating bad math.
Wikipedia tells us that, "In mathematics, two integers a and b are coprime, relatively prime or mutually prime if the only positive integer that is a divisor of both of them is 1.
As explained in the Test Guidance for the Affine Cipher, the list of valid, invertible values for a mod 26 is 1,3,5,7,9,11,15,17,19,21,23,25. This is the list of values that are coprime to 26, so it is incorrect to say, "Pick a value for a which is not coprime with 26".
I realize that the affine ciphers are not currently in use, but it still seems like a good idea to avoid propagating bad math.
Wikipedia tells us that, "In mathematics, two integers a and b are coprime, relatively prime or mutually prime if the only positive integer that is a divisor of both of them is 1.
As explained in the Test Guidance for the Affine Cipher, the list of valid, invertible values for a mod 26 is 1,3,5,7,9,11,15,17,19,21,23,25. This is the list of values that are coprime to 26, so it is incorrect to say, "Pick a value for a which is not coprime with 26".