- #1
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Homework Statement
In Calculus on Manifold pp.83-84, Spivak writes that "if v_1,...,v_{n-1} are vectors in R^n and f:R^n-->R is defined by f(w)=det(v_1,...,v_{n-1},w), then f is an alternating 1-tensor on R^n; therefore there is a unique z in R^n such that <w,z>=f(w) (and this z is denoted v_1 x ... x v_{n-1} and called the cross product of v_1,...,v_{n-1})."
I do not get how he makes that conclusion about the existence and uniqueness of z.
Existence is clear as such a z is easy to construct explicitly. But surely, z is not unique! Take for instance n=3, in which case <w,z>=|w||z|cosO, so that given a length |z| for z, it suffices to give it the angle O = arccos(f(w)/[|w||z|]) for the equality <w,z>=f(w) to hold, and evidently, there are an infinity of possible such lenght-angle combinations.