The doublestruck letter denotes the field of real numbers.
R
See also
C, I, N, Q, R--, R-+, R-n, Real Number, ZExplore with Wolfram|Alpha
References
Dummit, D. S. and Foote, R. M. Abstract Algebra, 2nd ed. Englewood Cliffs, NJ: Prentice-Hall, p. 1, 1998.Referenced on Wolfram|Alpha
RCite this as:
Weisstein, Eric W. "R." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/R.html