Apparently it's a common but not completely standardised notation for the two-point compactification of the real line: http://mathworld.wolfram.com/AffinelyExtendedRealNumbers.html Sincerely, Adam P. Goucher http://cp4space.wordpress.com

----- Original Message ----- From: James Cloos Sent: 09/22/13 07:09 PM To: math-fun@mailman.xmission.com Subject: [math-fun] notation query

I read a paper today which used a notation I do not remember seeing before; I'm curious whether it is common.

The paper discussed the union of the reals with a pair of points at infinity by using an overbar over the set R. It didn't explain the notation, but it was clear from the context.

For those with utf-8 support:

 ℝ̅ ≡ ℝ ⋃ { −∞, +∞ }

or in TeX:

 \bar{\mathbb{R}} \def \mathbb{R} \bigcup \{ -\inf, +\inf \}

or maybe \overling{\mathbb{R}}.

Is that common?

Thanks.

-JimC -- James Cloos <cloos@jhcloos.com> OpenPGP: 1024D/ED7DAEA6

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