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Number theory Difficulty 6.0 National Olympiad Prove it Canada

Problem:
Determine all positive integers aa, bb, cc, pp where pp and p+2p + 2 are odd primes and
2apb=(p+2)c1.2^{a}p^{b} = (p + 2)^{c} - 1.

Solution

Solution:
The only solution is (a,b,c,p)=(3,1,2,3)(a, b, c, p) = (3, 1, 2, 3). First, factor the right hand side. This gives us
2apb=(p+1)((p+2)c1+(p+2)c2++(p+2)+1).2^{a}p^{b} = (p + 1)((p + 2)^{c - 1} + (p + 2)^{c - 2} + \dots +(p + 2) + 1).
Since gcd(p,p+1)=1\gcd (p, p + 1) = 1 it must be the case that p+1=2xp + 1 = 2^{x} for some positive integer xax \leq a and so p=2x1p = 2^{x} - 1 and p+2=2x+1p + 2 = 2^{x} + 1. Now for x3x \geq 3, 2x+12^{x} + 1 is not prime if xx is odd (since it is 0mod30 \bmod 3) and 2x12^{x} - 1 is not prime if xx is even (since it is 0mod30 \bmod 3). This means x2x \leq 2, and the only admissible such xx is x=2x = 2 since otherwise pp is not prime. So, the original equation becomes
2a3b=5c1.2^{a}3^{b} = 5^{c} - 1.
Now 3(5c1)3|(5^{c} - 1), so evaluating 5c15^{c} - 1 mod 33 gives that c=2dc = 2d for some positive integer dd and hence
2a3b=(5d1)(5d+1).2^{a}3^{b} = (5^{d} - 1)(5^{d} + 1).
Observe 5d15^{d} - 1 and 5d+15^{d} + 1 are both even and have greatest common divisor 22 because they are 22 apart. Since 4(5d1)4|(5^{d} - 1) this implies 5d1=2a15^{d} - 1 = 2^{a - 1} and 5d+1=23b5^{d} + 1 = 2\cdot 3^{b}. Now, 33 is not a factor of 5d15^{d} - 1 because 5d1=2a15^{d} - 1 = 2^{a - 1}. Thus, by evaluating mod 33, dd must be odd. If d>1d > 1, this is impossible as 5d1=(51)(5d1+5d2++5+1)5^{d} - 1 = (5 - 1)(5^{d - 1} + 5^{d - 2} + \dots + 5 + 1) and the latter factor has an odd prime factor, contradicting 5d15^{d} - 1 is a power of 22. Thus d=1d = 1 and so c=2c = 2, implying that 2a3b=242^{a}3^{b} = 24 so a=3a = 3 and b=1b = 1. Hence, the only solution is (a,b,c,p)=(3,1,2,3)(a, b, c, p) = (3, 1, 2, 3).

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