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I was going through the Gravitation Lensing Wikipedia page and came across this formula: enter image description here

This notation of the Integral doesn't say anything to me.

Why is the expression after the $d$?

What does $d^2$ in this context mean?

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  • $\begingroup$ This is common notation in physics to denote an area integral. Similarly $d^3 x$ or something similar denotes a volume integral. $\endgroup$ – Alex Jun 22 '17 at 21:18
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$d^2\theta '$ means that you're integrating in two dimensions (it's a double integral, on both directions of $\theta'$.

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  • $\begingroup$ Oh. Thank you very much. $\endgroup$ – Miemels Jun 22 '17 at 21:46
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$ d^2 \theta^{'} = d\theta_x d\theta_y $

or something along these lines.

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