where and are modified Bessel functions.
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The term cylindrical harmonics is also used to refer to the Bessel functions (that are cylindrical harmonics in the sense described above).
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where and are ordinary Bessel functions.
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It can be seen that the Z(k,z) functions are the kernels of the Fourier transform or Laplace transform of the Z(z) function and so k may be a discrete variable for periodic boundary conditions, or it may be a continuous variable for non-periodic boundary conditions.