Marcum Q-function

In statistics, the Marcum-Q-function ${\displaystyle Q_{M}}$ is defined as

${\displaystyle Q_{M}}$ is also defined as

with modified Bessel function ${\displaystyle I_{M-1}}$ of order M − 1. The Marcum Q-function is used for example as a cumulative distribution function (more precisely, as a survivor function) for noncentral chi, noncentral chi-squared and Rice distributions.

...
Wikipedia