Give proof and make two questions related to the Theorem
Advanced Engineering Mathematics
10th Edition
ISBN:9780470458365
Author:Erwin Kreyszig
Publisher:Erwin Kreyszig
Chapter2: Second-order Linear Odes
Section: Chapter Questions
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Give proof and make two questions related to the Theorem

Transcribed Image Text:There are various methods for determining whether certain special types of
Mersenne numbers are prime or composite. One such test is presented next.
NUMBERS OF SPECIAL FORM
229
Theorem 11.3. If p and q = 2p+1 are primes, then either q |Mp or q| Mp+ 2, but
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)(2q-)/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
9q |Mp 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
9 = 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°)ª = 2°(-15)*(mod 47)
But
(-15)* = (225) = (-10)°
= 6 (mod 47)
Putting these two congruences together, we see that
223 = 23.6 = 48 = 1 (mod 47)
whence M23 is composite.
We might point out that Theorem 11.3 is of no help in testing the primality of
M29, say; in this instance, 59{ M29, but instead 59| M29 + 2.
Of the two possibilities q| M, or q| Mp+ 2, is it reasonable to ask: What
conditions on q will ensure that a LM.? The answer is to be found in Theorem 11.4.
Theorem 11.4. If g = 2n +l is prime, then we have the following:
7 (mod 8)
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