| definiendum | $\arctan: \{z\in\mathbb C\mid |z|\le 1, z\neq \pm i\}\to\mathbb C$ |
| definiendum | $\arctan(z):=\sum_{n=0}^\infty (-1)^n\frac{1}{2n+1} z^{2n+1} $ |
| $\frac{{\mathrm d}}{{\mathrm d}z}\arctan(z)=\frac{1}{1+z^2}$ |
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Wikipedia: Inverse trigonometric functions