no. 2
The Noether-Lefschetz theorem and sums of 4 squares in the rational function field R(x,y)
Compositio Mathematica, Tome 86 (1993) no. 2, pp. 235-243. 
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     author = {Colliot-Th\'el\`ene, J.-L.},
     title = {The {Noether-Lefschetz} theorem and sums of 4 squares in the rational function field $R(x, y)$},
     journal = {Compositio Mathematica},
     pages = {235--243},
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     number = {2},
     year = {1993},
     mrnumber = {1214459},
     zbl = {0774.12002},
     language = {en},
     url = {http://www.numdam.org/item/CM_1993__86_2_235_0/}
}
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Colliot-Thélène, J.-L. The Noether-Lefschetz theorem and sums of 4 squares in the rational function field $R(x, y)$. Compositio Mathematica, Tome 86 (1993) no. 2, pp. 235-243. http://www.numdam.org/item/CM_1993__86_2_235_0/

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