...textbooks

PVI or FGDP, for example, explain spectral factorization. More concisely in PVI, more richly in FGDP.

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...name

Any fact this basic should be named in some earlier field of mathematics or theoretical physics. Admittedly, the concept exists on an infinite cartesian plane without a helix, but all my codes in a finite space involve the helix, and the helix concept led me to it.

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...sometimes

Some programming minutia: Fortran77 does not allow you to refer to an array by both its cartesian coordinates and by its linear subscript in the same subroutine. To access it both ways, you need a subroutine call, or you dimension it as data(n1,n2,...) and then you refer to it as data(id,1,1,...). Fortran90 follows the same rule outside modules. Where modules use other modules, the compiler does not allow you to refer to data both ways, unless the array is declared as allocatable.

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