  For integer a let us consider the sequence X_a={x_0,x_1,x_2,...} defined by
x_0=a, x_1=1 and, for n>=1, x_{n+1}=x_n+x_{n-1}. We say that a prime p divides
X_a if p divides at least one term of the sequence. It is easy to see that
every prime p divides X_1, the sequence of Fibonacci numbers. Lagarias, using a
technique involving the computation of degrees of various Kummerian extensions
first employed by Hasse, showed in 1985 that X_2, the set of primes dividing
some Lucas number has natural density 2/3 and posed as a challenge finding the
density of prime divisors of X_3. In this paper we resolve this challenge,
assuming GRH, by showing that the density of X_3 equals 1573727S/1569610, with
S the so called Stephens constant. This is the first example of a `non-torsion'
second order recurrent sequence with irreducible recurrence relation for which
we can determine the associated density of prime divisors.
