1,3

a(4) and a(5) are determined by Odlyzko and Sloane, a(6) by Peters and Kok Seng Chua gives an explicit upper bound for all a(n). Also both a(7) and a(8) are either 2 or 3 as established by Chakraborty et al.

K. Chakraborty, A. K. Lal and B. Ramakrishnan, Modular forms that behave like theta series, Math. Computation, Vol. 66, 219, Jul 15 1997, pp. 1169-1183

Kok Seng Chua, An explicit Hecke's bound and exceptions of even unimodular quadratic forms, Bull. Austral. Math. Soc. 65 (2002), 231-238.

A. M. Odlyzko and N. J. A. Sloane, On exceptions of integral quadratic forms, J. reine angew Math. 321, 212-216, (1981)

M. Peters, Definite Unimodular 48-Dimensional Quadratic Forms, Bull. London Math. Soc., 15 (1983), 18-20

a(3)=2 because Leech lattice has no vectors of norm 2. All other 24-dimensional Niemeier lattices contains vectors of all even norms.

Sequence in context: A084954 A049300 A084957 this_sequence A004481 A004489 A112599

Adjacent sequences: A035304 A035305 A035306 this_sequence A035308 A035309 A035310

hard,nonn

Kok Seng Chua (chuaks(AT)ihpc.nus.edu.sg), May 25 2000