We use $h_K$ conventionally to denote the class numbers of number fields $K$.
But I have never thought why the letter $h$ was used for it.
Why and who used $h$ for the class number?
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I asked this question on MO once. The notation goes back to Dirichlet. See https://mathoverflow.net/questions/17062/why-is-h-the-notation-for-class-numbers