no. 1
A note on fields of definition
Compositio Mathematica, Tome 45 (1982) no. 1, pp. 109-113. 
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     author = {Karel, M.},
     title = {A note on fields of definition},
     journal = {Compositio Mathematica},
     pages = {109--113},
     publisher = {Martinus Nijhoff Publishers},
     volume = {45},
     number = {1},
     year = {1982},
     mrnumber = {648661},
     zbl = {0478.10014},
     language = {en},
     url = {http://www.numdam.org/item/CM_1982__45_1_109_0/}
}
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Karel, M. A note on fields of definition. Compositio Mathematica, Tome 45 (1982) no. 1, pp. 109-113. http://www.numdam.org/item/CM_1982__45_1_109_0/

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[2] J. Bogo and W. Kuyk: Hecke operators for G(√q), q prime; Eisenstein series and modular invariants. J. of Algebra 43(2), 583-605. | MR | Zbl

[3] M. Karel: Eisenstein series and fields of definition. Compositio Math. 37 (1978) 121-169. | EuDML | Numdam | MR | Zbl