mirror of
https://github.com/tgorordo/WignerSymbols.jl.git
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68 lines
3.9 KiB
Markdown
68 lines
3.9 KiB
Markdown
# WignerSymbols
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[](https://travis-ci.org/Jutho/WignerSymbols.jl)
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[](LICENSE.md)
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[](https://coveralls.io/github/Jutho/WignerSymbols.jl?branch=master)
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[](http://codecov.io/github/Jutho/WignerSymbols.jl?branch=master)
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Compute Wigner's 3j and 6j symbols, and related quantities such as Clebsch-Gordan
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coefficients and Racah's symbols.
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## Installation
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Install with the new package manager via `]add WignerSymbols` or
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```julia
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using Pkg
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Pkg.add("WignerSymbols")
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```
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## Available functions
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While the following function signatures are probably self-explanatory, you can query help
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for them in the Julia REPL to get further details.
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* `wigner3j(T::Type{<:Real} = RationalRoot{BigInt}, j₁, j₂, j₃, m₁, m₂, m₃ = -m₂-m₁) -> ::T`
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* `wigner6j(T::Type{<:Real} = RationalRoot{BigInt}, j₁, j₂, j₃, j₄, j₅, j₆) -> ::T`
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* `clebschgordan(T::Type{<:Real} = RationalRoot{BigInt}, j₁, m₁, j₂, m₂, j₃, m₃ = m₁+m₂) -> ::T`
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* `racahV(T::Type{<:Real} = RationalRoot{BigInt}, j₁, j₂, j₃, m₁, m₂, m₃ = -m₁-m₂) -> ::T`
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* `racahW(T::Type{<:Real} = RationalRoot{BigInt}, j₁, j₂, J, j₃, J₁₂, J₂₃) -> ::T`
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* `δ(j₁, j₂, j₃) -> ::Bool`
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* `Δ(T::Type{<:Real} = RationalRoot{BigInt}, j₁, j₂, j₃) -> ::T`
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The package relies on [HalfIntegers.jl](https://github.com/sostock/HalfIntegers.jl) to
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support and use arithmetic with half integer numbers, and, since v1.1, on
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[RationalRoots.jl](https://github.com/Jutho/RationalRoots.jl) to return the result exactly
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as the square root of a `Rational{BigInt}`, which will then be automatically converted to a
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suitable floating point value upon further arithmetic, using the `AbstractIrrational`
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interface from Julia Base.
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## Implementation
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Largely based on reading the paper (but not the code):
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[1] [H. T. Johansson and C. Forssén, SIAM Journal on Scientific Compututing 38 (2016) 376-384](https://doi.org/10.1137/15M1021908) ([arXiv:1504.08329](https://arxiv.org/abs/1504.08329))
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with some additional modifications to further improve efficiency for large `j` (angular
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momenta quantum numbers).
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In particular, 3j and 6j symbols are computed exactly, in the format `√(r) * s` where `r`
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and `s` are exactly computed as `Rational{BigInt}`, using an intermediate representation
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based on prime number factorization. As a consequence thereof, all of the above functions
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can be called requesting `BigFloat` precision for the result. There is currently no
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convenient syntax for obtaining `r` and `s` directly (see TODO).
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Most intermediate calculations (prime factorizations of numbers and their factorials,
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conversion between prime powers and `BigInt`s) are cached to improve the efficiency, but
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this can result in large use of memory when querying Wigner symbols for large values of `j`.
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Also uses ideas from
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[2] [J. Rasch and A. C. H. Yu, SIAM Journal on Scientific Compututing 25 (2003), 1416–1428](https://doi.org/10.1137/S1064827503422932)
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for caching the computed 3j and 6j symbols.
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## Todo
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* Wigner 9-j symbols, as explained in [1] and based on
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[3] [L. Wei, New formula for 9-j symbols and their direct calculation, Computers in Physics, 12 (1998), 632–634.](http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.481.5946&rep=rep1&type=pdf)
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* ~~Convenient syntax to get the exact results in the `√(r) * s` format, but in such a way
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that it can be used by the Julia type system and can be converted afterwards.~~
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Solved in v1.1 by the package RationalRoots.jl, the implementation of which was initiated by @w-vdh in [PR #9](https://github.com/Jutho/WignerSymbols.jl/pull/9).
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