Relating to Bessel functions or Bessel equations in mathematics and physics.
From 'Bessel' (the mathematician's name) + '-ian' (suffix making it an adjective). Standard academic terminology for concepts named after historical scientists.
In graduate-level physics courses, 'Besselian' is jargon for that moment when your differential equation suddenly gets complicated—it's a word mathematicians whisper with a mix of awe and resignation.
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