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Published online by Cambridge University Press: 02 December 2021
We show that geodesics in
$\mathbf {H}$
attached to a maximal split torus or a real quadratic torus in
$GL_{2, \mathbf {Q}}$
are the only irreducible algebraic curves in
$\mathbf {H}$
whose image in
$\mathbf {R}^2$
via the j-invariant is contained in an algebraic curve.
The author’s research is supported by the ERC Grant 804176.