The terms a(1)-a(12) are from the paper by Gibbs and McCranie.
The number of Ulam numbers not exceeding 10^n can be calculated from this sequence using the fact that up to 10^12 the only powers of 10 that are Ulam numbers are 10^6 and 10^11.
The asymptotic density D of Ulam numbers is known to be 0 <= D <= 0.4. Gibbs and McCranie estimated it from this sequence as ~ 0.073979.