Field induced antiferromagnetism and $^{17}$O Knight shift anomaly in La$_2$CuO$_4$

We address the effect of the field induced antiferromagnetism in paramagnetic state of the cuprate weak ferromagnet La$_2$CuO$_4$. The planar oxygen $^{17}$O Knight shift is shown to be an effective tool to inspect the effects of Dzyaloshinsky-Moriya coupling in cuprates in an external magnetic field. Field induced antiferromagnetism and anisotropic antiferromagnetic contribution to $^{17}$K explain the anomalies observed in $^{17}$O NMR in La$_2$CuO$_4$. The experimental observation of antiferromagnetic contribution to the $^{17}$O Knight shift provides probably the only way to find out the problem of the sense of Dzyaloshinsky vector in cuprates.