[e7b3d7b] | 1 | #fcc paracrystal model |
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[3271e20] | 2 | #note model title and parameter table are automatically inserted |
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| 3 | #note - calculation requires double precision |
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[e7b3d7b] | 4 | r""" |
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[b297ba9] | 5 | .. warning:: This model and this model description are under review following |
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| 6 | concerns raised by SasView users. If you need to use this model, |
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| 7 | please email help@sasview.org for the latest situation. *The |
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[da7b26b] | 8 | SasView Developers. September 2018.* |
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| 9 | |
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| 10 | Definition |
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| 11 | ---------- |
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| 12 | |
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[3c56da87] | 13 | Calculates the scattering from a **face-centered cubic lattice** with |
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| 14 | paracrystalline distortion. Thermal vibrations are considered to be |
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| 15 | negligible, and the size of the paracrystal is infinitely large. |
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| 16 | Paracrystalline distortion is assumed to be isotropic and characterized by |
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| 17 | a Gaussian distribution. |
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[3271e20] | 18 | |
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[d138d43] | 19 | The scattering intensity $I(q)$ is calculated as |
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| 20 | |
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| 21 | .. math:: |
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[3271e20] | 22 | |
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[eb69cce] | 23 | I(q) = \frac{\text{scale}}{V_p} V_\text{lattice} P(q) Z(q) |
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[3271e20] | 24 | |
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[d138d43] | 25 | where *scale* is the volume fraction of spheres, $V_p$ is the volume of |
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[eb69cce] | 26 | the primary particle, $V_\text{lattice}$ is a volume correction for the crystal |
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[d138d43] | 27 | structure, $P(q)$ is the form factor of the sphere (normalized), and $Z(q)$ |
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[3c56da87] | 28 | is the paracrystalline structure factor for a face-centered cubic structure. |
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[3271e20] | 29 | |
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[da7b26b] | 30 | Equation (1) of the 1990 reference\ [#CIT1990]_ is used to calculate $Z(q)$, |
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| 31 | using equations (23)-(25) from the 1987 paper\ [#CIT1987]_ for $Z1$, $Z2$, and |
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| 32 | $Z3$. |
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[3271e20] | 33 | |
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[3c56da87] | 34 | The lattice correction (the occupied volume of the lattice) for a |
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[eb69cce] | 35 | face-centered cubic structure of particles of radius $R$ and nearest |
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[d138d43] | 36 | neighbor separation $D$ is |
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[3271e20] | 37 | |
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[d138d43] | 38 | .. math:: |
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| 39 | |
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| 40 | V_\text{lattice} = \frac{16\pi}{3}\frac{R^3}{\left(D\sqrt{2}\right)^3} |
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[3271e20] | 41 | |
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[3c56da87] | 42 | The distortion factor (one standard deviation) of the paracrystal is |
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[d138d43] | 43 | included in the calculation of $Z(q)$ |
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| 44 | |
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| 45 | .. math:: |
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[3271e20] | 46 | |
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[d138d43] | 47 | \Delta a = gD |
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[3271e20] | 48 | |
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[d138d43] | 49 | where $g$ is a fractional distortion based on the nearest neighbor distance. |
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[3271e20] | 50 | |
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[2f0c07d] | 51 | .. figure:: img/fcc_geometry.jpg |
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[3271e20] | 52 | |
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[d138d43] | 53 | Face-centered cubic lattice. |
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[3271e20] | 54 | |
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| 55 | For a crystal, diffraction peaks appear at reduced q-values given by |
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| 56 | |
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[d138d43] | 57 | .. math:: |
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[3271e20] | 58 | |
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[d138d43] | 59 | \frac{qD}{2\pi} = \sqrt{h^2 + k^2 + l^2} |
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| 60 | |
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| 61 | where for a face-centered cubic lattice $h, k , l$ all odd or all |
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| 62 | even are allowed and reflections where $h, k, l$ are mixed odd/even |
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[3c56da87] | 63 | are forbidden. Thus the peak positions correspond to (just the first 5) |
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[3271e20] | 64 | |
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[d138d43] | 65 | .. math:: |
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| 66 | |
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| 67 | \begin{array}{cccccc} |
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| 68 | q/q_0 & 1 & \sqrt{4/3} & \sqrt{8/3} & \sqrt{11/3} & \sqrt{4} \\ |
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| 69 | \text{Indices} & (111) & (200) & (220) & (311) & (222) |
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| 70 | \end{array} |
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[3271e20] | 71 | |
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[eda8b30] | 72 | .. note:: |
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| 73 | |
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| 74 | The calculation of $Z(q)$ is a double numerical integral that |
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| 75 | must be carried out with a high density of points to properly capture |
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[1f159bd] | 76 | the sharp peaks of the paracrystalline scattering. |
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| 77 | So be warned that the calculation is slow. Fitting of any experimental data |
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[eda8b30] | 78 | must be resolution smeared for any meaningful fit. This makes a triple integral |
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| 79 | which may be very slow. |
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[3271e20] | 80 | |
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[eb69cce] | 81 | The 2D (Anisotropic model) is based on the reference below where $I(q)$ is |
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[3c56da87] | 82 | approximated for 1d scattering. Thus the scattering pattern for 2D may not |
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[1f159bd] | 83 | be accurate particularly at low $q$. For general details of the calculation |
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[eda8b30] | 84 | and angular dispersions for oriented particles see :ref:`orientation` . |
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| 85 | Note that we are not responsible for any incorrectness of the |
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[3c56da87] | 86 | 2D model computation. |
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[3271e20] | 87 | |
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[1f65db5] | 88 | .. figure:: img/parallelepiped_angle_definition.png |
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[d138d43] | 89 | |
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[404ebbd] | 90 | Orientation of the crystal with respect to the scattering plane, when |
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[1f65db5] | 91 | $\theta = \phi = 0$ the $c$ axis is along the beam direction (the $z$ axis). |
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[3271e20] | 92 | |
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[eb69cce] | 93 | References |
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| 94 | ---------- |
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[3271e20] | 95 | |
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[da7b26b] | 96 | .. [#CIT1987] Hideki Matsuoka et. al. *Physical Review B*, 36 (1987) 1754-1765 |
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| 97 | (Original Paper) |
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| 98 | .. [#CIT1990] Hideki Matsuoka et. al. *Physical Review B*, 41 (1990) 3854 -3856 |
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| 99 | (Corrections to FCC and BCC lattice structure calculation) |
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| 100 | |
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| 101 | Authorship and Verification |
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| 102 | --------------------------- |
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[3271e20] | 103 | |
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[da7b26b] | 104 | * **Author:** NIST IGOR/DANSE **Date:** pre 2010 |
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| 105 | * **Last Modified by:** Paul Butler **Date:** September 29, 2016 |
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| 106 | * **Last Reviewed by:** Richard Heenan **Date:** March 21, 2016 |
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[3271e20] | 107 | """ |
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| 108 | |
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[2d81cfe] | 109 | import numpy as np |
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[e2d6e3b] | 110 | from numpy import inf, pi |
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[3271e20] | 111 | |
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[e7b3d7b] | 112 | name = "fcc_paracrystal" |
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| 113 | title = "Face-centred cubic lattic with paracrystalline distortion" |
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[3271e20] | 114 | description = """ |
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[e7b3d7b] | 115 | Calculates the scattering from a **face-centered cubic lattice** with paracrystalline distortion. Thermal vibrations |
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[3271e20] | 116 | are considered to be negligible, and the size of the paracrystal is infinitely large. Paracrystalline distortion is |
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| 117 | assumed to be isotropic and characterized by a Gaussian distribution. |
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| 118 | """ |
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[a5d0d00] | 119 | category = "shape:paracrystal" |
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[3271e20] | 120 | |
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[13ed84c] | 121 | single = False |
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| 122 | |
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[c0ccea8] | 123 | # pylint: disable=bad-whitespace, line-too-long |
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[3e428ec] | 124 | # ["name", "units", default, [lower, upper], "type","description"], |
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| 125 | parameters = [["dnn", "Ang", 220, [-inf, inf], "", "Nearest neighbour distance"], |
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| 126 | ["d_factor", "", 0.06, [-inf, inf], "", "Paracrystal distortion factor"], |
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| 127 | ["radius", "Ang", 40, [0, inf], "volume", "Particle radius"], |
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[42356c8] | 128 | ["sld", "1e-6/Ang^2", 4, [-inf, inf], "sld", "Particle scattering length density"], |
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| 129 | ["sld_solvent", "1e-6/Ang^2", 1, [-inf, inf], "sld", "Solvent scattering length density"], |
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[9b79f29] | 130 | ["theta", "degrees", 60, [-360, 360], "orientation", "c axis to beam angle"], |
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| 131 | ["phi", "degrees", 60, [-360, 360], "orientation", "rotation about beam"], |
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| 132 | ["psi", "degrees", 60, [-360, 360], "orientation", "rotation about c axis"] |
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[3e428ec] | 133 | ] |
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[c0ccea8] | 134 | # pylint: enable=bad-whitespace, line-too-long |
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[3e428ec] | 135 | |
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[925ad6e] | 136 | source = ["lib/sas_3j1x_x.c", "lib/gauss150.c", "lib/sphere_form.c", "fcc_paracrystal.c"] |
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[3271e20] | 137 | |
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[404ebbd] | 138 | def random(): |
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[b297ba9] | 139 | """Return a random parameter set for the model.""" |
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[1511c37c] | 140 | # copied from bcc_paracrystal |
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| 141 | radius = 10**np.random.uniform(1.3, 4) |
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| 142 | d_factor = 10**np.random.uniform(-2, -0.7) # sigma_d in 0.01-0.7 |
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[404ebbd] | 143 | dnn_fraction = np.random.beta(a=10, b=1) |
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[8f04da4] | 144 | dnn = radius*4/np.sqrt(2)/dnn_fraction |
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[404ebbd] | 145 | pars = dict( |
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| 146 | #sld=1, sld_solvent=0, scale=1, background=1e-32, |
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[1511c37c] | 147 | dnn=dnn, |
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| 148 | d_factor=d_factor, |
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| 149 | radius=radius, |
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[404ebbd] | 150 | ) |
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| 151 | return pars |
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| 152 | |
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[69e1afc] | 153 | # april 10 2017, rkh add unit tests, NOT compared with any other calc method, assume correct! |
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[1f159bd] | 154 | # TODO: fix the 2d tests |
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[8f04da4] | 155 | q = 4.*pi/220. |
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[e2d6e3b] | 156 | tests = [ |
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[8f04da4] | 157 | [{}, [0.001, q, 0.215268], [0.275164706668, 5.7776842567, 0.00958167119232]], |
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[1f159bd] | 158 | #[{}, (-0.047, -0.007), 238.103096286], |
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| 159 | #[{}, (0.053, 0.063), 0.863609587796], |
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[e2d6e3b] | 160 | ] |
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