test is presented next. Theorem 11.3. If p and q = 2p+1 are primes, then either q | Mp or q | Mp + 2, but not both.

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Chapter10: Sequences, Series, And Probability
Section: Chapter Questions
Problem 63RE
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Prove Theorem 11.3
Of the two possibilities q | Mp or q| Mp + 2, is it reasonable to ask: What
Theorem 11.4. If q = 2n +1 is prime, then we have the following:
M9, say; in this instance, 59 M29, but instead 59 | M29 +2.
Theorem 11.3. If p and q =2p+1 are primes, then either q| M, or q| Mp+2, but
One such test is presented next.
types of
not both.
Proof. With reference to Fermat's theorem, we know that
29-1
-1=0 (mod q)
and, factoring the left-hand side, that
(24-1)/2 - 1)(24-1)/2 + 1) = (2P –
1)(2P+ 1)
= 0 (mod q)
What amounts to the same thing:
Mp(M, + 2) = 0 (mod q)
The stated conclusion now follows directly from Theorem 3.1. We cannot have both
g|M, and q | Mp+2, for then q| 2, which is impossible.
A single application should suffice to illustrate Theorem 11.3: if p = 23, then
q = 2p +1 = 47 is also a prime, so that we may consider the case of M23. The
question reduces to one of whether 47 | M23 or, to put it differently, whether 223
1 (mod 47). Now, we have
223 = 2 (2) = 23(-15)*(mod 47)
%3D
But
(-15) = (225) = (-10)² = 6 (mod 47)
Tuling these two congruences together, we see that
223 = 23.6 = 48 = 1 (mod 47)
whence M23 is composite.
e might point out that Theorem 11.3 is of no help in testing the primality of
Conditions on q
will ensure that g | M,? The answer is to be found in Theorem 11.4
(a)
9|M
(b) alM
provided that qg = 1 (mod 8) or q = 7 (mod 8).
O (mod 8) or a = 5 (mod 8).
Transcribed Image Text:Of the two possibilities q | Mp or q| Mp + 2, is it reasonable to ask: What Theorem 11.4. If q = 2n +1 is prime, then we have the following: M9, say; in this instance, 59 M29, but instead 59 | M29 +2. Theorem 11.3. If p and q =2p+1 are primes, then either q| M, or q| Mp+2, but One such test is presented next. types of not both. Proof. With reference to Fermat's theorem, we know that 29-1 -1=0 (mod q) and, factoring the left-hand side, that (24-1)/2 - 1)(24-1)/2 + 1) = (2P – 1)(2P+ 1) = 0 (mod q) What amounts to the same thing: Mp(M, + 2) = 0 (mod q) The stated conclusion now follows directly from Theorem 3.1. We cannot have both g|M, and q | Mp+2, for then q| 2, which is impossible. A single application should suffice to illustrate Theorem 11.3: if p = 23, then q = 2p +1 = 47 is also a prime, so that we may consider the case of M23. The question reduces to one of whether 47 | M23 or, to put it differently, whether 223 1 (mod 47). Now, we have 223 = 2 (2) = 23(-15)*(mod 47) %3D But (-15) = (225) = (-10)² = 6 (mod 47) Tuling these two congruences together, we see that 223 = 23.6 = 48 = 1 (mod 47) whence M23 is composite. e might point out that Theorem 11.3 is of no help in testing the primality of Conditions on q will ensure that g | M,? The answer is to be found in Theorem 11.4 (a) 9|M (b) alM provided that qg = 1 (mod 8) or q = 7 (mod 8). O (mod 8) or a = 5 (mod 8).
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