The constants C_n defined by C_n congruent [ integral_0^∞ left bracketing bar d/(d t) ((sin t)/t)^n right bracketing bar d t] - 1. These constants can also be written as the sums C_n = 2 sum_(k = 1)^∞ (1 + x_k^2)^(-n/2), and C_n = 2 sum_(k = 1)^∞ ( left bracketing bar sinc(x_k) right bracketing bar )^n (E. Weisstein, Feb. 3, 2015), where x_k is the kth positive root of t = tan t and sinc(x) is the sinc function.

series | tanc function