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The Bessel functions of the first kind  and  are defined as sums of the following infinite series:
These sums are convergent everywhere in the complex  ‐plane. The Bessel functions of the second kind  and  for noninteger parameter  are defined as special linear combinations of the last two functions:
In the case of integer index  , the right‐hand sides of the previous expressions give removable indeterminate values of the type  . In this case, the Bessel functions  and  are defined through the following limits:
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