Table of n, a(n) for n=0..8.
R. S. Nickerson, Problem E1845, Amer. Math. Monthly, p. 81, 73 (1966).
R. S. Nickerson and D. C. B. Marsh, E1845: A variant of Langford's Problem, American Math. Monthly, 1967, 74, 591-595.
R. J. Nowakowski, Generalizations of the Langford-Skolem problem, M.S. Thesis, Dept. Math., Univ. Calgary, May 1975. [Scanned copy, with permission.]