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I have the following expression

BesselJ[\[ImaginaryI]*m, k*r]

FunctionExpand knows how to transform from HankelH1 to BesselJ, but not the other way around (see comment by @flinty ).

How can I force Mathematica expressiosn using BesselJ in terms of HankelH1 and HankelH2?

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  • $\begingroup$ Mathematica doesn't want to FunctionExpand the Bessel functions into Hankel functions, but it works the other way round if you look at the documentation for HankelH1/H2. We have: HankelH1[I m, k r] == BesselJ[I*m, k*r] + I BesselY[I*m, k*r] and HankelH2[I m, k r] == BesselJ[I m, k r] - I BesselY[I m, k r] and so therefore FullSimplify[HankelH1[I m, k r] + HankelH2[I m, k r]]/2 gives BesselJ[I m, k r] $\endgroup$
    – flinty
    Dec 3 '21 at 12:08
  • $\begingroup$ One could have expected that FullSimplify would accept options like TargetFunctions->{HankelH1,HankelH2}, but it doesn't. $\endgroup$
    – rhermans
    Dec 3 '21 at 13:07
  • $\begingroup$ Might check what W|A can do: wolframalpha.com/input/?i=convert+besselj+to+Hankel $\endgroup$ Dec 3 '21 at 20:41
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For a simple expression that doesn't require simplification, it should be enough to use ReplaceAll with a suitable substitution (see the answer by @BobHanlon for a way to deduce this rule programmatically). I define B2H to be the operator form of ReplaceAll,

 B2H = ReplaceAll[
    {
      BesselJ[a_, b_] :>   (HankelH1[a, b] + HankelH2[a, b])/2,
      BesselY[a_, b_] :> I*(HankelH2[a, b] - HankelH1[a, b])/2
    }
 ]

BesselJ[\[ImaginaryI]*m, k*r]  //B2H
(*(HankelH1[\[ImaginaryI] m,k r]+HankelH2[\[ImaginaryI] m,k r])/2 *)

But if you need to use FullSimplify on a more complex expression, then you will need to explicitly force BesselJ and BesselY to be expensive functions (using ComplexityFunction), and offer a way to go from Bessel to Hankel functions, like before, (by using TransformationFunctions), so the following should work.

We create our own ComplexityFunction by taking the regular SimplifyCount and adding a large number for expression with BesselJ or BesselY, similar to the example in the documentation for ComplexityFunction

myCF=Function[{e}, 
          100 Count[e, _BesselJ|_BesselY, {0, Infinity}]
        + Simplify`SimplifyCount[e]
     ];

Now we can call FullSimplify using myCF and B2H as customized options.

FullSimplify[
    BesselJ[\[ImaginaryI]*m, k*r]
    ,ComplexityFunction->myCF
    ,TransformationFunctions->{Automatic,B2H}
]
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Clear["Global`*"]

Solve for Bessel functions in terms of Hankel functions

sol = Solve[(# == FunctionExpand[#]) & /@ {HankelH1[n, z], 
    HankelH2[n, z]}, {BesselJ[n, z], BesselY[n, z]}]

(* {{BesselJ[n, z] -> 1/2 (HankelH1[n, z] + HankelH2[n, z]), 
  BesselY[n, z] -> -(1/2) I (HankelH1[n, z] - HankelH2[n, z])}} *)

Convert solutions to generalized replacement rules

rulesBtoH = Rule @@@ Thread[{
    (sol[[1, All, 1]] /. {n :> n_, z :> z_}),
    sol[[1, All, 2]]}]

(* {BesselJ[n_, z_] -> 1/2 (HankelH1[n, z] + HankelH2[n, z]), 
 BesselY[n_, z_] -> -(1/2) I (HankelH1[n, z] - HankelH2[n, z])} *)

Use replacement rules for conversion

BesselJ[I*m, k*r] /. rulesBtoH

(* 1/2 (HankelH1[I m, k r] + HankelH2[I m, k r]) *)
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