  We use the representation theory of Lie algebras and computational linear
algebra to determine the simplest nonconstant invariant polynomial in the
entries of a general 2 x 2 x 3 array. This polynomial is homogeneous of degree
6 and has 66 terms with coefficients 1, -1, 2, -2 in the 12 indeterminates
x_ijk where i,j = 1,2 and k = 1,2,3. This invariant can be regarded as a
natural generalization of Cayley's hyperdeterminant for 2 x 2 x 2 arrays.
