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## 3.2 Mathematical Constants

Constants are represented as functions with no arguments and must be called as such. For instance, the mathematical constant $$\pi$$ must be written in a Stan program as pi().

real pi()
$$\pi$$, the ratio of a circle’s circumference to its diameter

real e()
$$e$$, the base of the natural logarithm

real sqrt2()
The square root of 2

real log2()
The natural logarithm of 2

real log10()
The natural logarithm of 10