The number $\pi$ (read pi) is defined as twice the uniquely defined zero of the cosine function in the closed interval $[0,2].$
$$\cos\left(\frac\pi2\right)=0\quad\Longleftrightarrow\quad\frac\pi2=1.570796326\pm 10^{-9}\quad\Longleftrightarrow\quad\pi\approx 3.141592653\pm 10^{-9}.$$
Corollaries: 1 2 3 4
Definitions: 5
Lemmas: 6
Proofs: 7 8
Propositions: 9 10 11 12 13
Theorems: 14
