The text accompanying the figure of addition/subtraction on an elliptic curve includes the following statement: The reflection of this point in the x-axis will give defines the result of "addition", -R = P+Q, in the abelian group of rational solutions of the elliptic curve.

Either "will give" or "defines" should be omitted -- probably the former.

And, what a wonderful and approachable article. Thank you.

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