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$\mathbb{R}$, written as \mathbb{R} in MathJax, is the letter R in the special font "blackboard bold". Traditionally, several letters in this font have special meanings: $\mathbb{R}$ real numbers $\mathbb{Z}$ integers $\mathbb{N}$ natural numbers $\mathbb{Q}$ rational numbers (fractions) $\mathbb{C}$ complex numbers

Near the beginning of a class for which Dantzig was late, professor Jerzy Neyman wrote two examples of famously unsolved statistics problems on the blackboard. When Dantzig arrived, he assumed that the two problems were a homework assignment and wrote them down.