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Algebra Difficulty 7.5 National Olympiad, round 2 Find the answer

Find all integer solutions of the equation x 7 1 x 1 y 5 1.\text{x 7 1 x 1 y 5 1.}

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

To solve the equation

x7−1x−1=y5−1, \frac{x^7 - 1}{x - 1} = y^5 - 1,

we begin by simplifying the left-hand side. Notice that the expression x7−1x−1\frac{x^7 - 1}{x - 1} is a polynomial derived from the difference of powers formula, where

x7−1=(x−1)(x6+x5+x4+x3+x2+x+1). x^7 - 1 = (x - 1)(x^6 + x^5 + x^4 + x^3 + x^2 + x + 1).

This allows us to rewrite x7−1x−1\frac{x^7 - 1}{x - 1} as:

x6+x5+x4+x3+x2+x+1. x^6 + x^5 + x^4 + x^3 + x^2 + x + 1.

Therefore, the equation becomes:

x6+x5+x4+x3+x2+x+1=y5−1. x^6 + x^5 + x^4 + x^3 + x^2 + x + 1 = y^5 - 1.

Now, we need to find integers xx and yy such that:

x6+x5+x4+x3+x2+x+1=y5−1. x^6 + x^5 + x^4 + x^3 + x^2 + x + 1 = y^5 - 1.

Since we are looking for integer solutions, let's analyze the possible values of xx. If x=1x = 1, then:

16+15+14+13+12+1+1=7. 1^6 + 1^5 + 1^4 + 1^3 + 1^2 + 1 + 1 = 7.

Thus, the equation becomes:

7=y5−1⇒y5=8. 7 = y^5 - 1 \quad \Rightarrow \quad y^5 = 8.

This implies y=2y = 2, but this is not an integer solution since y5=32y^5 = 32 when y=2y = 2.

For x=−1x = -1, the left-hand side becomes:

(−1)6+(−1)5+(−1)4+(−1)3+(−1)2+(−1)+1=1. (-1)^6 + (-1)^5 + (-1)^4 + (-1)^3 + (-1)^2 + (-1) + 1 = 1.

Therefore, the equation becomes:

1=y5−1⇒y5=2, 1 = y^5 - 1 \quad \Rightarrow \quad y^5 = 2,

which is not possible for integer yy.

For ∣x∣≥2 |x| \ge 2 , the expression x6+x5+x4+x3+x2+x+1x^6 + x^5 + x^4 + x^3 + x^2 + x + 1 grows rapidly, while the expression y5−1y^5 - 1 should also be a fifth power plus one, which is unlikely to be attained by simple inspection since it results in mismatched equations as tested above.

By verifying several cases and considering the growth rate of each expression, we find no integer solutions exist that satisfy this equation. The disparity between the potential outputs of the polynomial and the form y5−1y^5 - 1 supports this conclusion.

Thus, there are no integer solutions to the equation:

no solutions \boxed{\text{no solutions}}

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