0,2

The terms eventually becomes negative, and so for large n the extremal codes do not exist (see references, also the extended sequence in the first link).

Up to n = 250 the terms steadily increase in magnitude, but their sign changes from positive to negative at n = 154.

F. J. MacWilliams and N. J. A. Sloane, The Theory of Error-Correcting Codes, Elsevier-North Holland, 1978, see Theorem 13, p. 624.

C. L. Mallows and N. J. A. Sloane, An Upper Bound for Self-Dual Codes, Information and Control, 22 (1973), 188-200.

N. J. A. Sloane, Table of n, a(n) for n = 0..250

G. Nebe, E. M. Rains and N. J. A. Sloane, Self-Dual Codes and Invariant Theory, Springer, Berlin, 2006.

E. M. Rains and N. J. A. Sloane, Self-dual codes, pp. 177-294 of Handbook of Coding Theory, Elsevier, 1998 (Abstract, pdf, ps).

N. J. A. Sloane, My favorite integer sequences, in Sequences and their Applications (Proceedings of SETA '98).

At length 24, the answer is 1+759*x^8+2576*x^12+..., with leading coefficient 759.

For Maple program see A034414.

Cf. A034414 (leading coefficient), A001380, A034597, A034598.

Sequence in context: A031789 A001294 A109026 this_sequence A035894 A096298 A033695

Adjacent sequences: A034412 A034413 A034414 this_sequence A034416 A034417 A034418

nonn

njas