I understand that $b=Y_n(a)$ is the Bessel Function of the second kind, and the Bessel Function of the second kind swaps between negative and positive values for $b$ repeatedly as $a$ increases, for positive values of $a$. Also there is a modified Bessel Function of the second kind $b=K_n(a)$, in which $b$ is positive for all positive values of $a$, and decreases as $a$ increases.
I tried putting $b=Y_n(ia)$ into wolfram alpha but it did not get interpreted as the Bessel Function of the second kind with the imaginary unit.
How do I express $b=Y_n(ia)$ another way?