It is well known that for $K=\mathbb{Q}(\sqrt{D})$, $D < 0$, the non-maximal order of squarefree conductor $f$, relatively prime to $D$, has class number $$h_K \prod_{p|f} (p-(\frac{D}{p}))$$

What is the class number of a non-maximal order in an imaginary quadratic extension of $\mathbb{F}_p[t]$? Is it proven using ideas involving zeta function, as appears in many books for the case above?

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    $\begingroup$ EGA IV$_4$, 2nd line of ( $\endgroup$ – BCnrd Jul 15 '10 at 19:03
  • $\begingroup$ So practically the same thing... Thanks! $\endgroup$ – Dror Speiser Jul 15 '10 at 20:18
  • $\begingroup$ I was actually looking for a proof of the original fact you quoted, and Googling led me here... would you please provide a reference? Thank you! $\endgroup$ – Frank Thorne Feb 25 '11 at 4:48
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    $\begingroup$ p.s. Please, I beg you, not EGA. $\endgroup$ – Frank Thorne Feb 25 '11 at 4:55
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    $\begingroup$ Harvey Cohn's Advanced Number Theory, chapter 13, section 2, theorem 2 (Dover edition, pp. 217). $\endgroup$ – Dror Speiser Feb 25 '11 at 7:49

There is a formula that works in all degrees, not just imaginary quadratic. In a global field $K$, let $O$ be integral over ${\mathbf Z}$ or ${\mathbf F}[t]$ (${\mathbf F}$ a finite field) and be "big", i.e., it has fraction field $K$. Let $\mathfrak c$ be the conductor ideal of $O$ in its integral closure $R$. Then $$h(O) = \frac{h(R)}{[R^\times:O^\times]}\frac{\varphi_{R}({\mathfrak c})}{\varphi_O(\mathfrak c)},$$ where $\varphi_O(\mathfrak c)$ is the number of units in $O/\mathfrak c$ and $\varphi_R(\mathfrak c)$ is the number of units in $R/\mathfrak c$. This is derived in Neukirch's alg. number theory book in the number field case, but it goes through to any one-dimensional Noetherian domain with a finite residue rings and a finite class group. In the imag. quadratic case the unit index $[R^\times:O^\times]$ is 1 most of the time so you don't notice it.

Both $\varphi_R(\mathfrak c)$ and $\varphi_O(\mathfrak c)$ can be written in the form ${\text N}(\mathfrak c)\prod_{\mathfrak p \supset \mathfrak c}(1 - 1/{\text{N}(\mathfrak p)})$, where the ideal norm $\text N$ means the index in $R$ or $O$ and $\mathfrak p$ runs over primes in $R$ or $O$ for the two cases.

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