OFFSET
1,1
COMMENTS
Next term, if it exists, is greater than 10^850. - Sascha Kurz, Sep 22 2003
No more terms < 10^20000. - David Wasserman, Feb 08 2005
The problem of whether there are any other terms in this sequence, Brocard's problem, has been unsolved since 1876. The known calculations give a(4) > (10^9)! = factorial(10^9). - Stefan Steinerberger, Mar 19 2006
I wrote a similar program sieving against the 40 smallest primes larger than 4*10^9 and can report that a(4) > factorial(4*10^9+1). In other words, it's now known that the only n <= 4*10^9 for which n!+1 is a square are 4, 5 and 7. C source code available on request. - Tim Peters (tim.one(AT)comcast.net), Jul 02 2006
Robert Matson claims to have verified that 4, 5, and 7 are the only values of n <= 10^12 for which n!+1 is a square. This implies that the next term, if it exists, is greater than (10^12+1)! ~ 1.4*10^11565705518115. - David Radcliffe, Oct 28 2019
REFERENCES
R. Guy, "Unsolved Problems in Number Theory", 3rd edition, D25
Clifford A. Pickover, A Passion for Mathematics (2005) at 69, 306.
LINKS
Bruce C. Berndt and William F. Galway, On the Brocard-Ramanujan Diophantine Equation n! + 1 = m^2, The Ramanujan Journal, March 2000, Volume 4, Issue 1, pp 41-42.
Robert D. Matson, Brocard's Problem 4th Solution Search Utilizing Quadratic Residues, Unsolved Problems.
Wikipedia, Brocard's problem
FORMULA
EXAMPLE
5^2 = 25 = 4! + 1;
11^2 = 121 = 5! + 1;
71^2 = 5041 = 7! + 1.
PROG
(PARI) A085692=select( issquare, vector(99, n, n!+1)) \\ M. F. Hasler, Nov 20 2018
CROSSREFS
KEYWORD
nonn,bref
AUTHOR
Klaus Strassburger (strass(AT)ddfi.uni-duesseldorf.de), Jul 18 2003
STATUS
approved