Do integers, n,m, exist such that n!+1=m2 other than n=4,5,7?
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
Dabrowski (1996) generalized Overholt's result by showing that it would follow from the abc conjecture that
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
has only finitely many integer solutions for a given polynomial P(x) of degree at least 2 with integer coefficients.
- Berndt, Bruce C.; Galway, William F. (2000), "The Brocard–Ramanujan diophantine equation n! + 1 = m2" (PDF), The Ramanujan Journal 4: 41–42, doi:10.1023/A:1009873805276.
- Brocard, H. (1876), "Question 166", Nouv. Corres. Math. 2: 287.
- Brocard, H. (1885), "Question 1532", Nouv. Ann. Math. 4: 391.
- Dabrowski, A. (1996), "On the Diophantine Equation x! + A = y2", Nieuw Arch. Wisk. 14: 321–324.
- Guy, R. K. (1994), "D25: Equations Involving Factorial", Unsolved Problems in Number Theory (2nd ed.), New York: Springer-Verlag, pp. 193–194, ISBN 0-387-90593-6.
- Luca, Florian (2002), "The diophantine equation P(x) = n! and a result of M. Overholt" (PDF), Glasnik Matematički 37 (57): 269–273.
- Overholt, Marius (1993), "The diophantine equation n! + 1 = m2", Bull. London Math. Soc. 25 (2): 104, doi:10.1112/blms/25.2.104.