There are gaps in the lines above and below, just to the left of the "symbol", that make me think something was left out in the picture. That is, that only part is shown. That, together with the captions, make me think it is simply an "R" with the vertical line missing. And that, of course, means the set of real numbers.
Last edited by HallsofIvy; Jan 24th 2011 at 11:00 AM.