# Relatively prime determinants

Suppose you fill two n×n matrices with random integers.

What is the probability that the determinants of the two matrices are relatively prime? By “random integers” we mean that the integers are chosen from a finite interval, and we take the limit as the size of the interval grows to encompass all integers.

Let Δ(n) be the probability that two random integer matrices of size n have relatively prime determinants.

The function Δ(n) is a strictly decreasing function of n.

The value of Δ(1) is known exactly.

It is the probability that two random integers are relatively prime, which is well known to be 6/π².

The limit of Δ(n) as n goes to infinity is known as the Hafner-Sarnak-McCurley constant [1], which has been computed to be 0.

3532363719… Since Δ(n) is a decreasing function, the limit is also a lower bound for all n.

Python simulation Here is some Python code to experiment with the math discussed above.

We’ll first do a simulation to show that we get close to 6/π² for the proportion of relatively prime pairs of integers.

Then we look at random 2×2 determinants.

from sympy import gcd from numpy.

random import randint from numpy import pi def coprime(a, b): return gcd(a, b) == 1 def random_int(N): return randint(-N, N) def random_det(N): a, b, c, d = randint(-N, N, 4) return a*d – b*c count = 0 N = 10000000 # draw integers from [-N, N) num_reps = 1000000 for _ in range(num_reps): count += coprime(random_int(N), random_int(N)) print(“Simulation: “, count/num_reps) print(“Theory: “, 6*pi**-2) This code printed Simulation: 0.

607757 Theory: 0.

6079271018540267 when I ran it, so our simulation agreed with theory to three figures, the most you could expect from 106 repetitions.

The analogous code for 2×2 matrices introduces a function random_det.

def random_det(N): a, b, c, d = randint(-N, N, 4, dtype=int64) return a*d – b*c I specified the dtype because the default is to use (32 bit) int as the type, which lead to Python complaining “RuntimeWarning: overflow encountered in long_scalars”.

I replaced random_int with random_det and reran the code above.

This produced 0.

452042.

The exact value isn’t known in closed form, but we can see that it is between the bounds Δ(1) = 0.

6079 and Δ(∞) = 0.

3532.

Theory In [1] the authors show that This expression is only known to have a closed form when n = 1.

Related posts Probability of coprime sets Perpendicular and relatively prime [1] Hafner, J.

L.

; Sarnak, P.

& McCurley, K.

(1993), “Relatively Prime Values of Polynomials”, in Knopp, M.

& Seingorn, M.

(eds.

), A Tribute to Emil Grosswald: Number Theory and Related Analysis, Providence, RI: Amer.

Math.

Soc.

, ISBN 0-8218-5155-1.

.