2007 USAMO Problems/Problem 5

Problem

(Titu Andreescu) Prove that for every nonnegative integer $n$ , the number $7^{7^n}+1$ is the product of at least $2n+3$ (not necessarily distinct) primes.

Solutions

Solution 1

The proof is by induction. The base is provided by the $n = 0$ case, where $7^{7^0} + 1 = 7^1 + 1 = 2^3$ . To prove the inductive step, it suffices to show that if $x = 7^{2m - 1}$ for some positive integer $m$ then $(x^7 + 1)/(x + 1)$ is composite. As a consequence, $x^7 + 1$ has at least two more prime factors than does $x + 1$ . To confirm that $(x^7 + 1)/(x + 1)$ is composite, observe that $\begin{align*} \frac{x^7 + 1}{x + 1} &= \frac{(x + 1)^7 - ((x + 1)^7 - (x^7 + 1))}{x + 1} \\ &= (x + 1)^6 - \frac{7x(x^5 + 3x^4 + 5x^3 + 5x^2 + 3x + 1)}{x + 1} \\ &= (x + 1)^6 - 7x(x^4 + 2x^3 + 3x^2 + 2x + 1) \\ &= (x + 1)^6 - 7^{2m}(x^2 + x + 1)^2 \\ &= \{(x + 1)^3 - 7^m(x^2 + x + 1)\}\{(x + 1)^3 + 7^m(x^2 + x + 1)\}. \end{align*}$ Also each factor exceeds 1. It suffices to check the smaller one; $\sqrt{7x}\leq x$ gives $\begin{align*} (x + 1)^3 - 7^m(x^2 + x + 1) &= (x + 1)^3 - \sqrt{7x}(x^2 + x + 1) \\ &\geq x^3 + 3x^2 + 3x + 1 - x(x^2 + x + 1) \\ &= 2x^2 + 2x + 1\geq 113 > 1. \end{align*}$ Hence $(x^7 + 1)/(x + 1)$ is composite and the proof is complete.

Solution 2

We prove by induction on $n$ that $7^{7^n} + 1$ is the product of at least $2n + 3$ (not necessarily distinct) primes.

For $n = 0$ , we have $7^{7^0} + 1 = 7 + 1 = 8 = 2^3$ , which has $3 = 2(0) + 3$ prime factors. Thus, the base case holds.

Assume that for some $n \ge 0$ , the number $7^{7^n} + 1$ is the product of at least $2n + 3$ primes. We must show that $7^{7^{n+1}} + 1$ has at least $2n + 5$ prime factors.

Let $a = 7^n$ . Since $a$ is odd, we can apply the Aurifeuillian factorization: $7^{7a} + 1 = (7^a + 1)\big(7^{3a} + 3\cdot7^{2a} + 3\cdot7^a + 1\big)\big(7^{\tfrac{5a}{2} + \tfrac{1}{2}} + 7^{\tfrac{3a}{2} + \tfrac{1}{2}} + 7^{\tfrac{a}{2} + \tfrac{1}{2}}\big)$ .

Each factor is an integer greater than 1, so the factorization is nontrivial.

The first factor is $7^a + 1 = 7^{7^n} + 1$ , which by the inductive hypothesis has at least $2n + 3$ prime factors. It remains to show that the quotient $\frac{7^{7a} + 1}{7^a + 1}$ is composite.

From the factorization above, we have $\frac{7^{7a} + 1}{7^a + 1} = \big(7^{3a} + 3\cdot7^{2a} + 3\cdot7^a + 1\big)\big(7^{\tfrac{5a}{2} + \tfrac{1}{2}} + 7^{\tfrac{3a}{2} + \tfrac{1}{2}} + 7^{\tfrac{a}{2} + \tfrac{1}{2}}\big)$ ,

which is clearly the product of two integers greater than 1, and thus composite.

Therefore, $\frac{7^{7a} + 1}{7^a + 1}$ is always composite, and hence $7^{7a} + 1$ has at least two more prime factors than $7^a + 1$ .

By the inductive hypothesis, $7^a + 1$ has at least $2n + 3$ prime factors, so $7^{7a} + 1$ has at least $(2n + 3) + 2 = 2n + 5$ prime factors.

By induction, $7^{7^n} + 1$ is the product of at least $2n + 3$ (not necessarily distinct) primes for all nonnegative integers $n$ .

Alternate solutions are always welcome. If you have a different, elegant solution to this problem, please add it to this page.

2007 USAMO (Problems • Resources)
Preceded by Problem 4	Followed by Problem 6
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2007 USAMO Problems/Problem 5

Contents

Problem

Solutions

Solution 1

Solution 2

See also