Equations for Lower Bounds on Border Rank

We present new methods for determining polynomials in the ideal of the variety of bilinear maps of border rank at most r. We apply these methods to several cases including the case r = 6 in the space of bilinear maps C × C → C. This space of bilinear maps includes the matrix multiplication operator M2 for two by two matrices. We show these newly obtained polynomials do not vanish on the matrix multiplication operator M2, which gives a new proof that the border rank of the multiplication of 2× 2 matrices is seven. Other examples are considered along with an explanation of how to implement the methods. Acknowledgements. We thank Peter Bürgisser for important discussions and suggestions, and the anonymous reviewer for many helpful comments.