I'm a bit confused with how fractions are handled with adding points of elliptic curves over finite fields. Below is an example from the text which I am trying to understand:

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The part that particularly confuses me is

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If I think about this the same way that $1/5$ was handled, we have that $246/1 = 246\cdot1^{-1}=246\cdot1=246$, but $246\equiv12 \mod 13$ so shouldn't $246/1 = 12$? I don't understand how the calculation is being done. Could someone explain?

  • 2
    $\begingroup$ Hmm, the text seems to be in error. $\endgroup$
    – anon
    Commented Jul 22, 2013 at 6:34
  • $\begingroup$ @anon math.stackexchange.com/questions/430836/… referring to this post my calculation seems to make sense, so you are probably right $\endgroup$
    – Alex
    Commented Jul 22, 2013 at 6:43

1 Answer 1


You are right. The text is just missing a minus sign. The answer is $$\lambda \equiv 12 \equiv -1 \mod 13.$$


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