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Published online by Cambridge University Press: 20 November 2018
If p is a polynomial in one real variable and p(x) = p(-x) then p has only even powers of x and is thus a polynomial in x 2. If p is a polynomial in n variables and p(x 1, …, x n ) = p(y 1, …, y n ) when x 1 2 + … + x n 2 = y 1 2+ … + y n 2 then p is a polynomial in q where q(x 1, …, x n ) = x 1 2 + … + x n 2.