Krall and Fink defined the Bessel polynomials as the function y_n(x) | = | sum_(k = 0)^n ((n + k)!)/((n - k)!k!) (x/2)^k | = | sqrt(2/(π x))e^(1/x) K_(-n - 1/2)(1/x), where K_n(x) is a modified Bessel function of the second kind. They are very similar to the modified spherical bessel function of the second kind k_n(x).

Bessel function | modified spherical Bessel function of the second kind | Sheffer sequence