I am studying Bessel modified $K_\nu$ functions, and I do not find any clear reference for the following question: is there a relation between $K_\nu(x)$ and $K_\mu(x)$ in general?
More specifically, I would like to know the relation (which can be merely bounds or asymptotics) between $K_{a+it}(x)$ and $K_{it}(x)$.