MATH
midterm-solutions

# This will be w question why do we choose a decoding

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This will be W . Question: Why do we choose a decoding exponent d such that de 1 (mod φ ( n ))? In particular, what theorem must we use to prove that this is the correct decoding exponent, and why? Solution. The reason for this choice of decoding exponent is that decoding should re- verse encoding: that is, after encoding and then decoding we should get back the original message. Encoding and then decoding corresponds to raising the original message W to the de power and reducing (mod n ). Euler’s theorem allows us to ignore multiples of φ ( n ) in the exponent; de is one more than a multiple of φ ( n ), and so W de W (mod n ). 10. We saw two proofs that 2 is irrational. Explain why one of these proofs fails to 3

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show that 4 is irrational. (The answer should be specific to one of the proofs we saw in class; the observation that 4 is rational, by itself, does not suffice.) Solution. In one proof that we gave, we wrote 2 = c/d , which we rearranged to get c 2 = 2 d 2 . We then observed that c 2 is divisible by 2 and so c must be divisible by 2, which is why the proof worked. But if c 2 is divisible by 4, then c is not necessarily divisible by 4. In the other proof we gave, we considered the prime factorizations of c and d in c 2 = 2 d 2 . On the left-hand side, there must be an even number of twos in the prime factorization; on the right-hand side there must be an odd number of twos. But this does not work when 2 is replaced with 4 because now both sides have an even number of twos. END OF EXAM 4
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• Summer '09
• Lugo
• Math, Solution., Bank identification number, prime factorizations

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