SymArrays.jl

This package provides some tools to efficiently store arrays with exchange symmetries, i.e., arrays where exchanging two indices leaves the value unchanged. It stores the underlying data in a flat vector and provides mappings that allow to address it as a "normal" AbstractArray{T,N}. To generate a new one with undefined data, use

S = SymArray{Nsyms,T}(dims...)

where NSyms is a tuple that indicates the size of each group of exchangeable indices (which have to be adjacent for simplicity), T is the element type (e.g., Float64 or ComplexF64), and dims are the dimensions of the array (which have to fulfill length(dims)==sum(Nsyms). As an example

S = SymArray{(3,1,2,1),Float64}(10,10,10,3,50,50,50)

declares an array S[(i,j,k),l,(m,n),o] where any permutation of (i,j,k) leaves the value unchanged, as does any permutation of (m,n). Note that interchangeable indices obviously have to have the same size.

TODO:

• Allow specification and treatment of Hermitian indices, where any permutation conjugates the result (possibly only for 2 indices at a time?).