5
$\begingroup$

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:

enter image description here

The part that particularly confuses me is

enter image description here

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?

$\endgroup$
  • 2
    $\begingroup$ Hmm, the text seems to be in error. $\endgroup$ – anon Jul 22 '13 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 Jul 22 '13 at 6:43
3
$\begingroup$

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

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.