Brocard's problem

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Not to be confused with Brocard's conjecture.

Brocard's problem is a problem in mathematics that asks to find integer values of n for which

n!+1 = m^2,

where n! is the factorial. It was posed by Henri Brocard in a pair of articles in 1876 and 1885, and independently in 1913 by Srinivasa Ramanujan.

Question dropshade.png Unsolved problem in mathematics:
Do integers, n,m, exist such that n!+1=m2 other than n=4,5,7?
(more unsolved problems in mathematics)

Brown numbers[edit]

Pairs of the numbers (n, m) that solve Brocard's problem are called Brown numbers. There are only three known pairs of Brown numbers:

(4,5), (5,11), and (7,71).

Paul Erdős conjectured that no other solutions exist. Overholt (1993) showed that there are only finitely many solutions provided that the abc conjecture is true. Berndt & Galway (2000) performed calculations for n up to 109 and found no further solutions.

Variants of the problem[edit]

Dabrowski (1996) generalized Overholt's result by showing that it would follow from the abc conjecture that

n!+A = k^2

has only finitely many solutions, for any given integer A. This result was further generalized by Luca (2002), who showed (again assuming the abc conjecture) that the equation

n! = P(x)

has only finitely many integer solutions for a given polynomial P(x) of degree at least 2 with integer coefficients.


External links[edit]