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For a function $f$ that maps set $A$ to $B$,

  • $f\colon\mathbb R^+\to\mathbb R^+$, $f(x) = x^2$ is injective.
  • $f\colon\mathbb R\to\mathbb R$, $f(x) = x^2$ is not injective since $(- x)^2 = x^2$.

what is the difference between $\mathbb R^+$ and $\mathbb R$?

Additionally, what is the difference between $\mathbb N$ and $\mathbb N^+$?

Asaf Karagila
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James
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2 Answers2

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$\mathbb R^+$ commonly denotes the set of positive real numbers, that is: $$\mathbb R^+ = \{x\in\mathbb R\mid x>0\}$$

It is also denoted by $\mathbb R^{>0},\mathbb R_+$ and so on.

For $\mathbb N$ and $\mathbb N^+$ the difference is similar, however it may be non-existent if you define $0\notin\mathbb N$. In many set theory books $0$ is a natural number, while in analysis it is often not considered a natural number. Your mileage may vary on $\mathbb N$ vs. $\mathbb N^+$.

Asaf Karagila
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  • I see now. I thought it meant that R+ included some extra element. This interpretation makes much more sense. – James Aug 26 '12 at 20:02
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    Note that $\mathbb N^+ = \mathbb Z^+$. Also if you want to confuse your readers, you can write the empty set as $\mathbb N^-$, the set of negative natural numbers. :-) – celtschk Aug 26 '12 at 20:40
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    NB : Depending on the country, $\mathbb R^+$ is also used for the set of non-negative real numbers. – Student Aug 26 '12 at 22:15
  • @Student: Really? $0\in\mathbb R^+$? Sounds bizarre! – Asaf Karagila Aug 26 '12 at 22:38
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    @Asaf: as Ranabir says in a separate answer, it makes sense for $\mathbb{R}^+$ to be the set of positive reals. In some countries "positive" includes zero. Compare Willie Wong's answer to http://math.stackexchange.com/questions/26705/is-zero-positive-or-negative – Carl Mummert Aug 27 '12 at 00:45
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    @Carl: Read the first line, I also say it defines the positive numbers. I just never thought zero is positive... :-) (Also, these French just have to do everything the other way around...! :-)) – Asaf Karagila Aug 27 '12 at 01:19
  • I was taught that $\mathbb{R}^+$ is the set of positive real numbers greater than $0$ and $\mathbb{R}_+ = {(x, y)\in\mathbb{R} : x,y\geqslant 0}$. – Mr Pie Jan 26 '18 at 07:59
  • @user477343: I didn't know that the elements of $\Bbb R$ are intervals, or ordered pairs... Since they are not, $\Bbb R_+$ as you defined it would be just $\varnothing$. – Asaf Karagila Jan 26 '18 at 08:00
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    Go here $\longrightarrow$ https://math.stackexchange.com/questions/263175/what-does-the-notation-mathbb-r2-mean?rq=1 – Mr Pie Jan 26 '18 at 08:02
  • @user477343: If only there was a difference between $\Bbb R$ and $\Bbb R^2$... – Asaf Karagila Jan 26 '18 at 08:03
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    It implies for all $\mathbb{R}^n$. – Mr Pie Jan 26 '18 at 08:03
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    @user477343: I don't understand you, and your notation is not consistent. So I'm going to stop replying now. Have a great day! – Asaf Karagila Jan 26 '18 at 08:04
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Simply $\mathbb R$ means the set of real numbers.

$\mathbb R^+$ means the set of positive real numbers.

And $\mathbb R^-$ means the set of negative real numbers.

Argha
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