

A008350


Number of orbits of norm 2n vectors in E_8 lattice.


1



1, 1, 1, 1, 2, 1, 1, 2, 2, 2, 2, 2, 2, 3, 2, 2, 4, 3, 3, 4, 3, 3, 4, 4, 3, 5, 4, 5, 6, 5, 3, 6, 6, 5, 6, 6, 6, 8, 6, 6, 7, 7, 6, 10, 8, 7, 8, 9, 7, 10, 9, 9, 11, 11, 8, 11, 10, 10, 12, 12, 9, 13, 11, 13, 14, 13, 10, 17, 14, 12, 13, 15, 13, 17, 15, 15, 17, 18, 13, 19, 16, 16, 18, 21, 15, 20, 18, 19
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OFFSET

0,5


COMMENTS

Since Aut(E8) is a reflection group one can compute this using nonnegative combinations of the basis dual to the simple roots, since these are the lattice vectors in a fundamental domain and so include a unique representative of each orbit.  Noam D. Elkies, Apr 07 2008


REFERENCES

J. H. Conway and N. J. A. Sloane, "Sphere Packings, Lattices and Groups", SpringerVerlag, p. 123.


LINKS

N. J. A. Sloane, Table of n, a(n) for n = 0..1200 [Computed using Elkies's PARI/GP program]
J. H. Conway and N. J. A. Sloane, LowDimensional Lattices VII: Coordination Sequences, Proc. Royal Soc. London, A453 (1997), 23692389 (pdf).
Gabriele Nebe and N. J. A. Sloane, Home page for this lattice


PROG

(PARI)
M = 2*matid(8); for(i=1, 6, M[i, i+1] = M[i+1, i] = 1); M[3, 8] = M[8, 3] = 1; \\ M is now the Gram matrix for the simple roots of E8
M = 1/M; \\ M is now the Gram matrix for the dual basis to the simple roots; their nonnegative combinations are a fundamental domain for W(E8)
{ orbit_counts(N) =
c = vector(N);
v = vector(8, n, 0);
j = 1;
while(j<9, j = 1; v[1]++; k = v*M*v~; while(k>2*N, v[j]=0; j++; if(j<9, v[j]++; k=v*M*v~, k=0)); if(k, c[k/2]++); );
return(concat(1, c)) }
orbit_counts(100) \\ Noam D. Elkies, Apr 07 2008


CROSSREFS

Sequence in context: A256945 A282091 A015718 * A019556 A165640 A082892
Adjacent sequences: A008347 A008348 A008349 * A008351 A008352 A008353


KEYWORD

nonn


AUTHOR

N. J. A. Sloane and J. H. Conway


EXTENSIONS

Corrected by Noam D. Elkies, Apr 07 2008


STATUS

approved



