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what is the font used to designate Z (ring of integers), Q (field of rationals), R (field of reals) and C (field of complex)?

jack vogt
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    Welcome to TeX.SX. Merlin has given you the answer down below - but just to give you a bit of info, this style is referred to as "blackboard bold" and not everybody is a fan. The idea behind blackboard bold is that it's how bold letters can be written on a blackboard. In other words, these are sometimes still (and originally always were) just plain old bold letters and when they were written on a blackboard, they were double-struck to mimic the emboldening. So you can (and people do) just use bold as well. https://en.wikipedia.org/wiki/Blackboard_bold – Au101 Jul 26 '15 at 00:24

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You can use $\mathbb{R}$ for this and include the amssymb package: \usepackage{amssymb}. Also consider this: Real number symbol (UTF-8: ℝ) in XeTeX

Community
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Merlin1896
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Old style books (and Bourbaki up to nowadays) used simply 

   \mathbf Z, \mathbf Q, \mathbf R, \mathbf C

As explained by @Au101, the use of blackboardbold faces has its origin in the quasi-impossibility to write boldface letters on a blackboard, and their conventional replacement with double-struck letters

Bernard
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