no. 1
A remark about isogenies of elliptic curves over quadratic fields
Compositio Mathematica, Tome 58 (1986) no. 1, pp. 133-134. 
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     author = {Frey, Gerhard},
     title = {A remark about isogenies of elliptic curves over quadratic fields},
     journal = {Compositio Mathematica},
     pages = {133--134},
     publisher = {Martinus Nijhoff Publishers},
     volume = {58},
     number = {1},
     year = {1986},
     mrnumber = {834050},
     zbl = {0596.14021},
     language = {en},
     url = {http://www.numdam.org/item/CM_1986__58_1_133_0/}
}
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Frey, Gerhard. A remark about isogenies of elliptic curves over quadratic fields. Compositio Mathematica, Tome 58 (1986) no. 1, pp. 133-134. http://www.numdam.org/item/CM_1986__58_1_133_0/

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