At the bottom of this page on the Riemann zeta function, the following constants are defined in equations (133), (137), and (141), respectively:
$$C_{1} := \sum_{n=2}^{\infty} \frac{\zeta(n)}{n!}, $$ $$C_{2} := \sum_{n=1}^{\infty} \frac{\zeta(2n)}{n!}, $$ and $$C_{3} := \sum_{n=1}^{\infty} \frac{...