Clarify INumber.IsPositive and INumber.IsNegative documentation

[JochCool]

The method name and documentation of INumber<TSelf>.IsPositive(TSelf) implied that it would only return true for positive numbers, therefore excluding zero. However, all implementations of this interface do return true for zero.

I also clarified that these methods only return true for real numbers, as complex numbers cannot be categorised into strictly "positive" and "negative".

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Issue Details

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The method name and documentation of INumber<TSelf>.IsPositive(TSelf) implied that it would only return true for positive numbers, therefore excluding zero. However, all implementations of this interface do return true for zero.

I also clarified that these methods only return true for real numbers, as complex numbers cannot be categorised into strictly "positive" and "negative".

Author:	JochCool
Assignees:	-
Labels:	area-System.Numerics, community-contribution
Milestone:	-

[tannergooding]

You'll also need to update the docs here https://github.com/dotnet/dotnet-api-docs/blob/main/xml/System.Numerics/INumberBase%601.xml

It's worth noting that in the mathematics world whether or not zero is considered positive is up to some debate and may depend on your field, specialization, or even country of origin. There are sides that view it as neither positive nor negative and sides that view it as both positive and negative. Likewise, due to it being a natural number there are cases where math allows it to be included in the positive domain but where it may still be treated differently from numbers that are viewed as strictly positive.

That all being said, in the computer programming world and particularly due to things like the IEEE 754 standard, the existence of one's complement numbers, error introduced by rounding, and even for performance reasons it is frequently considered that zero is positive (or more specifically 0 and +0 are positive, where-as -0 is negative) .

[dnfadmin]

All CLA requirements met.

[JochCool]

Yes, I'll open a pull request for the other repository once the phrasing is finalised here.

And yeah there are other definitions of "positive", but excluding zero is by far the most widespread, at least in Western education. And in any case, when multiple different definitions are present, it is always worth clarifying which definition is being used.

[dakersnar]

I do think this wording update is clearer. LGTM.