MHB Finding the Minimum $x$ for a Prime Product

Click For Summary
The discussion focuses on determining the minimum natural number $x$ such that the expression $x^2 - x + 11$ can be expressed as a product of four primes. Participants explore various values of $x$ and calculate the resulting expressions to identify valid prime factorizations. The conversation includes testing different integers and analyzing the outcomes to find the smallest suitable $x$. Ultimately, the goal is to find the least $x$ that meets the criteria of producing a product of four primes. The challenge combines elements of number theory and prime factorization.
anemone
Gold Member
MHB
POTW Director
Messages
3,851
Reaction score
115
Find the minimum value for $x$ where $x\in \mathbb{N} $ and $x^2-x+11$ can be written as a product of 4 primes, which are not necessarily distinct.
 
Mathematics news on Phys.org
The key question to ask is: which primes can occur as factors of $N = x^2 - x + 11$, where $x$ is an integer?
If $x$ is an integer then either $x$ or $x-1$ is even, so $x^2-x = x(x-1)$ is always even and therefore $N = x^2 - x + 11$ is always odd. Thus the prime $2$ can never be a divisor of $N$.

Can the prime $3$ occur as a divisor of $N$? If so then (working mod $3$) $$0 \equiv x^2 - x + 11 \equiv x^2 + 2x + 2 \equiv (x+1)^2 + 1 \pmod3.$$ Therefore $(x+1)^2 \equiv -1\equiv 2 \pmod3$. But that equation has no solutions. In the language of number theory, $2$ is not a quadratic residue mod $3$. Therefore $3$ can never be a prime factor of $N$.

Can the prime $5$ occur as a divisor of $N$? If so then $0 \equiv x^2 - x + 11 \equiv x^2 + 4x + 1 \equiv (x+2)^2 + 2 \pmod5$. Therefore $(x+2)^2\equiv -2\equiv3 \pmod5$. But $3$ is not a quadratic residue mod $5$. Therefore $5$ can never be a prime factor of $N$.

Can the prime $7$ occur as a divisor of $N$? If so, then $0 \equiv x^2 - x + 11 \equiv x^2 + 6x + 4 \equiv (x+3)^2 + 2 \pmod7$. Therefore $(x+3)^2\equiv -2\equiv5 \pmod7$. But $3$ is not a quadratic residue mod $7$. Therefore $7$ can never be a prime factor of $N$.

But the prime $11$ can occur as a divisor of $N$. So can the prime $13$. So the smallest possible candidate for $N$ to be a product of four primes would be if $N = 11^4 = 14641$. However, if $x = 121$ then $x^2 - x + 11 = 121*120 + 11 = 14531 < 14641$, but if $x = 122$ then $x^2 - x + 11 = 122*121 + 11 = 14773 > 14641$. So $11^4$ falls between two values of $x^2 - x + 11$ and therefore does not give a solution to the problem.

The next smallest possible candidate for $N$ is $11^3*13 = 17303$. This time we strike lucky, because if $x=132$ then $x^2 - x + 11 = 132*131 + 11 = 17303$. So the minimum value for $x$ such that $x^2 - x + 11$ is a product of four primes is $x = 132$, and the primes are then $11,11,11,13$.
 
Here is a little puzzle from the book 100 Geometric Games by Pierre Berloquin. The side of a small square is one meter long and the side of a larger square one and a half meters long. One vertex of the large square is at the center of the small square. The side of the large square cuts two sides of the small square into one- third parts and two-thirds parts. What is the area where the squares overlap?

Similar threads

  • · Replies 9 ·
Replies
9
Views
2K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 3 ·
Replies
3
Views
1K
  • · Replies 5 ·
Replies
5
Views
2K
  • · Replies 3 ·
Replies
3
Views
1K
  • · Replies 10 ·
Replies
10
Views
3K
  • · Replies 0 ·
Replies
0
Views
1K
  • · Replies 4 ·
Replies
4
Views
2K
  • · Replies 7 ·
Replies
7
Views
2K
  • · Replies 24 ·
Replies
24
Views
3K