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Marc Palm
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There is a Cartan decomposition for $PGL_2(\mathbb{Q}_p)$ meaning that double coset of the from $PGL_2(\mathbb{Q}_p) //PGL_2(\mathbb{Z}_p)$ are represented by $\alpha^k$, $k\geq0$.

So this is the reason because the definitions are equivalent.

Marc Palm
  • 11.2k
  • 2
  • 35
  • 92