$X_{\text{ns}}^+(N)$ is $X_H$ for $H\le \GL_2(\widehat\Z)$ the inverse image of an extended Cartan subgroup of $\GL_2(\Z/N\Z)$ that is nonsplit at every prime dividing $N$.

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- Last edited by Andrew Sutherland on 2023-07-09 09:04:08

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