The Noether-Lefschetz theorem and sums of 4 squares in the rational function field $R\left(x,y\right)$
Compositio Mathematica, Volume 86 (1993) no. 2, pp. 235-243.
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title = {The {Noether-Lefschetz} theorem and sums of 4 squares in the rational function field $R(x, y)$},
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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, Volume 86 (1993) no. 2, pp. 235-243. http://www.numdam.org/item/CM_1993__86_2_235_0/`

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