Let $N \geq 5$ be a prime, and $H$ a subgroup of $GL\_2(\mathbb{F}\_N)$. As shown in Chapter IV of [DeRap], there is a curve $X\_H(N)$, defined over $K\_N := \mathbb{Q}(\zeta\_N)^{\det H}$, which is a coarse moduli space of elliptic curves with **level-$H$ structure**; that is, given any $L$ an extension of $K\_N$, the $L$-points of $X\_H(N)$ correspond to $\bar{L}$-isomorphism classes of elliptic curves over $L$ whose associated mod-$N$ representation on $N$-torsion points is contained in (a conjugate of) $H$. (Sometimes this is a *fine* moduli space, but I don't think this is important for this question). 


It is certainly *not* the case that, if $E/L$ has level-$H$ structure at $N$, then so too do all twists of $E$. For example, if $H$ were the subgroup $\left\{\left(\begin{array}{cc}1 & \ast \\ 0 & \ast \end{array}\right)\right\}$, corresponding to $X\_1(N)$, then it is not true.


But this *is* true sometimes; e.g., if $H = \left\{\left(\begin{array}{cc}\ast & \ast \\ 0 & \ast \end{array}\right)\right\}$, the Borel subgroup; this follows from the slogan: **if an elliptic curve has a rational cyclic isogeny, then so too do all twists**. A similar argument sows this for $H$ being the normaliser of a split Cartan subgroup, viz $\left\{\left(\begin{array}{cc}\ast & 0 \\ 0 & \ast \end{array}\right)\right\} \bigcup \left\{\left(\begin{array}{cc}0 & \ast \\ \ast & 0 \end{array}\right)\right\}$. I'll say that these level structures are **twist-invariant**. 


This leads me to ask:



> 
> Is there a characterisation of the twist-invariant level structures? What is it about a subgroup $H$ that makes it twist-invariant?
> 
> 
> 


[DeRap] : P. Deligne, M. Rapoport, ``Les schemas de modules de courbes elliptiques''.



Which level structures on elliptic curves are twist-invariant?
The criterion is that H contains -I.


