We often encounter interesting constants as:
- solutions to interesting equations
- values of special functions at special inputs
- integrals of differential forms on geometric objects
The third case is possibly viewable as a special case of the first. None of these are necessarily constrained to be real. Here are some examples:
- Small-order roots of unity (complex or p-adic) have plenty of number-theoretic utility, along with roots of interesting non-cyclotomic polynomials (e.g., $x^p-x-1/p$ in the p-adic world). We get $2\pi i$ by choosing a generator of the kernel of the exponential map $\mathbb{C} \to \mathbb{C}$, i.e., a special solution to the equation $e^z = 1$. Integers in imaginary quadratic fields can be viewed as locations of poles of special Weierstrass functions, and the zeroes of the Riemann zeta function are reasonably interesting, although perhaps not as isolated examples.
- Constants like $e$ and $\gamma$ arise as values of special functions, namely $e^x$ and $-\Gamma'(x)$ at $x=1$. We can get similar constants in other rings by evaluating special functions in those domains, or taking residues at poles. For example, certain modular functions evaluated at imaginary quadratic integers yield algebraic integers whose degree depends on class number.
- We can also think of $2 \pi i$ as the integral of $dz/z$ along a 1-cycle in $\mathbb{C}^\times$. More complicated constants arise from integrals on cycles in more complicated varieties. These are known as periods, and they exist in both complex and nonarchimedean worlds.
