But
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Build all or part of an inter-spectral or transfer matrix from functions or a table of functions.

When the user chooses to enter a series of functions, they are used in full. When the user chooses to enter a table_function, the stored functions can be split, averaged and windowed, with a given overlap.

To clarify the point, we assume that the user has 2 functions :math:`A` and :math:`B`. If he chooses to use the operator from functions, the definition of the interspectrum :math:`{G}_{\mathrm{AB}}` that he will calculate will necessarily be

:math:`{G}_{\mathrm{AB}}=\frac{1}{{N}_{A}}\mathrm{FFT}(A)\ast \mathrm{FFT}{(B)}^{\mathrm{H.}}`.

Where :math:`{N}_{A}` is the number of points in the :math:`A` vector. If he chooses to build a table_function with the functions :math:`A` and :math:`B`, he can extract :math:`N` under vectors :math:`{A}_{\mathrm{1,}}\mathrm{...},{A}_{N}` from the vector :math:`A` (resp. :math:`{B}_{\mathrm{1,}}\dots ,{B}_{N}` from the vector :math:`B`), and calculate an averaged interspectrum :math:`{G}_{\mathrm{AB}}` such that

:math:`{G}_{\mathrm{AB}}=\frac{1}{{N}_{A}}(\frac{1}{N}\sum _{k=1}^{N}\mathrm{FFT}({A}_{k})\ast \mathrm{FFT}{({B}_{k})}^{\mathrm{H.}})`.

The functions read are of the sd_function type.

The product concept is of an interspectrum type.

It is possible to use this operator in a *Code_Aster,* command file but also interactively using the operator CALC_ESSAI (U4.90.01).