OFFSET
2,1
COMMENTS
Also the largest prime that divides either n-1 or n+1.
Størmer shows that a(n) tends to infinity with n. Schinzel shows that lim inf a(n)/log log n >= 2 and, using lower bounds for linear forms of logarithms, this inequality can be generalized for general quadratic polynomials, with 2 replaced by 4/7 for irreducible ones and 2/7 for reducible ones. - Tomohiro Yamada, Apr 15 2017
REFERENCES
K. Mahler, "Uber den grossten Primteiler spezieller Polynome zweiten Grades", Arch. Math. Naturvid. B.41, 1935, pp. 3 - 26.
LINKS
T. D. Noe, Table of n, a(n) for n = 2..10000
D. H. Lehmer, On a problem of Størmer, Ill. J. Math., 8 (1964), 57-79.
A. Schinzel, On two theorems of Gelfond and some of their applications, Acta Arith. 13 (1967), 177--236.
Carl Størmer, Quelques théorèmes sur l'équation de Pell x^2 - Dy^2 = +-1 et leurs applications (in French), Skrifter udgivne af Videnskabsselskabet i Christiania: Mathematisk-naturvidenskabelig Klasse, 2 (1897), 48 pp.
EXAMPLE
n=11: the largest prime factor of 10 and 12 is 5, therefore a(11) = 5.
MATHEMATICA
Table[ Last[ Table[ # [[1]]] & /@ FactorInteger[n^2 - 1]], {n, 2, 80}]
PROG
(PARI) for (n=3, 100, print1(", "max(factor(n-1)[, 1][length(factor(n-1)[, 1])], factor(n+1)[, 1][length(factor(n+1)[, 1])])))
CROSSREFS
KEYWORD
nonn
AUTHOR
Jon Perry, Oct 21 2002
STATUS
approved