<math> D_0 \approx \frac{2 \left[ \frac{\text{cos} \left( \frac{k_0 L}{2} \text{cos} \theta \right) - \text{cos} \left( \frac{k_0 L}{2} \right) }{\text{sin}\theta} \right]^2 } {\gamma + \text{ln}(k_0L) - C_i(k_0L) + \frac{1}{2} \text{sin}(k_0L) \left[ S_i(2k_0L) - 2S_i(k_0L) \right] + \frac{1}{2} \text{cos}(k_0L) \left[ \gamma + \text{ln}(k_0L/2) + C_i(2k_0L) - 2C_i(k_0L) \right] } </math>
<math> F_1(x) = \gamma + \text{ln}(x) - C_i(x) </math>
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<math> F_2(x) = \frac{1}{2} \text{sin}(x) \left[ S_i(2x) - 2S_i(x) \right] </math>
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<math> F_3(x) = \frac{1}{2} \text{cos}(x) \left[ \gamma + \text{ln}(x/2) + C_i(2x) - 2C_i(x) \right] </math>
where γ = 0.5772 is the Euler-Mascheroni constant, and C<sub>i</sub>(x) and S<sub>i</sub>(x) are the cosine and sine integrals, respectively: