Using instanton homology with coefficients in Z/2 we construct a
homomorphism q2 from the homology cobordism group in dimension 3 to the
integers which is not a rational linear combination of the instanton
h--invariant and the Heegaard Floer correction term d. If an oriented
homology 3--sphere Y bounds a smooth, compact, negative definite
4--manifold without 2--torsion in its homology then q2(Y)≥0, with
strict inequality if the intersection form is non-standard.Comment: 88 pages, 2 figure