Hacker News Reader

Comment by carloswilson | Hacker News Reader

> Also, this post conflates the unary negation operator with negative numbers.

-a is a standard way to represent additive inverse of an element in field.

The point about "unary negation operator" seems irrelevant.

In the real number field, additive inverse of a positive real number is indeed the negative of that number. The negative of that number is also obtained by the application of unary negation operator on the positive number.

The additive inverse of 3.14 is -3.14. Unary negation operator applied to 3.14 gives us -3.14. I don't see how conflating unary negation operator with negative numbers here is any issue here.

View on HN

> In the real number field, additive inverse of a positive real number is indeed the negative of that number. The negative of that number is also obtained by the application of unary negation operator on the positive number.

This is a fact that follows from the definition of +. But + needs to be defined before you can start making assumptions about what the additive inverse is. The set over which the field is defined (Z or R) already contains -3, -2 etc. and -3 * -2 or -3 + -2 needs to be defined when you're constructing the field. It then turns out that -3 is the additive inverse of 3. You can't use this when arguing about the definition of why applying * on negative 2 and negative 3 gives you the result positive 6. Because you need to define * over all members of the field before you construct a field in the first place.

> The set over which the field is defined (Z or R)

What? The set of all integers, Z, is not a field! R is. But Z isn't. Z is a ring, a commutative ring. I doubt you understand what a field is!

> already contains -3, -2 etc.

Yes, and those elements are literally the additive inverses of their positive counterparts. If you disagree with this, then the numbers -3, -2, etc. literally have no meaning.

> It then turns out that -3 is the additive inverse of 3.

Are you making this all up with your original research or do you have any proper literature written by a professional mathematician to back it up?

To be fair, you can have a field that only has integers. For example, Z mod 5 is a field.

I am aware. It is typically represented as Z_5. They are called prime fields.

I highly doubt wsxcde meant prime fields in their comment though. wsxcde seemed to be talking about the set of all integers and the set of all real numbers in their comment. Only the latter is a field (and a ring) whereas the former is only a ring.

And (-a)(-b) = ab holds in rings (and thus fields).

Reply on news.ycombinator.com