... lensing

Appendices A and B are only available in electronic form at http://www.edpsciences.org

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...$\lambda$

The asymptotic expansion of the diffraction integral has been studied in optics. See the following text books for a detailed discussion: Kline & Kay (1965), Chap. XII; Mandel & Wolf (1995), Chap. 3.3; Born & Wolf (1997), Appendix III, and references therein.

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...$0 < \alpha \leq 2$)

We do not consider the steeper profile $\alpha > 2$, since the mass at the lens center is infinite.

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