Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

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?

share|cite|improve this question
Hmm, the text seems to be in error. – anon Jul 22 '13 at 6:34
@anon… referring to this post my calculation seems to make sense, so you are probably right – Alex Jul 22 '13 at 6:43
up vote 1 down vote accepted

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

share|cite|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

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