# Weird mathematical symbol that looks like a plus sign

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http://www.devmaster.net/wiki/Ray-triangle_intersection

In "P = O + distance dot D" I'd think it's a plus sign, except a vector plus a scalar makes no sense and it looks "off" from what I'd expect of a plus sign. That, and given that it's followed by a dot operation, I've got a few of what are probably facts:

A) It takes a vector and a scalar (the operation for distance cannot result in a vector).
B) It results in a vector.
C) It's done before the dot operation (as "scalar dot vector" also makes no sense).

I could be wrong, though.

Thanks.

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SiCrane    11839
No, it's a plus sign. Remember operator precedence is multiplication before addition.

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Quote:
 Original post by SiCraneNo, it's a plus sign. Remember operator precedence is multiplication before addition.

So you can dot a scalar by a vector and add a scalar to a vector? Because unless you can, I don't see how it could be a plus sign.

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SiCrane    11839
A scalar times a vector is a vector. So then you're adding a point and a vector.

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owl    376
Quote:
Original post by Narf the Mouse
Quote:
 Original post by SiCraneNo, it's a plus sign. Remember operator precedence is multiplication before addition.

So you can dot a scalar by a vector and add a scalar to a vector? Because unless you can, I don't see how it could be a plus sign.

I'm not a geniou but I think it is vector-scalar multiplication and then vector addition...

EDIT: It's an honor being beaten by SiCrane. lol

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Quote:
 Original post by SiCraneA scalar times a vector is a vector. So then you're adding a point and a vector.

Ah, thanks. I saw the dot and assumed it meant a vector dot operation, not multiplication. I'm used to either a multiplication sign or adjacency being used to designate multiplication.

And I suppose vector-scalar multiplication is technically a dot operation, too, except it doesn't act on the same number of values for both sides.

Bah. Mathematicians need to standardize. :D

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alvaro    21265
That notation is standard enough.

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Quote:
 Original post by alvaroThat notation is standard enough.

"...Operates on two vectors of the same length or a vector and scalar" would be easy to make standard. Yes, it's on the Wikipedia page. I've also seen it stated elsewise elsewhere. So if there is a standard (which can't be Wikipedia, as a standard requires a standards body or group) then it isn't one of: fully known, understood or accepted.

My point stands.

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alvaro    21265
Well, I was a mathematician for a few years, and I would say that notation is not confusing at all. There is no organism that defines standard notations for Math, and one is not needed. Different fields use different notations because they find them useful, and this is a good thing.

The reason for this thread is that you are not familiar enough with linear algebra and you had trouble understanding the expression you found on that page, but that is not a symptom of any larger problems in the Math community.

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alvaro    21265
Quote:
 Original post by Narf the Mouse"...Operates on two vectors of the same length or a vector and scalar" would be easy to make standard.

You have to be careful with language when doing Math: Vectors don't always have a "length", and when they do, it's not what you mean here.