The FunctionAdvisor(classify_function, math_function) command returns the class(es) to which the given function belongs.

BesselK belongs to the subclass "Bessel_related" of the class "0F1" and so, in principle, it can be related to various of the 26 functions of those classes - see FunctionAdvisor( "Bessel_related" ); and FunctionAdvisor( "0F1" );

tan belongs to the subclass "trig" of the class "elementary" and so, in principle, it can be related to various of the 26 functions of those classes - see FunctionAdvisor( "trig" ); and FunctionAdvisor( "elementary" );

JacobiTheta1 belongs to the class "Jacobi_related" and so, in principle, it can be related to various of the 18 functions of that class - see FunctionAdvisor( "Jacobi_related" );