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subroutine pda_cfftb(n,c,wsave)
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subroutine pda_cfftb computes the backward complex discrete fourier
transform (the fourier synthesis). equivalently , pda_cfftb computes
a complex periodic sequence from its fourier coefficients.
the transform is defined below at output parameter c.
a call of pda_cfftf followed by a call of pda_cfftb will multiply the
sequence by n.
the array wsave which is used by subroutine pda_cfftb must be
initialized by calling subroutine pda_cffti(n,wsave).
input parameters
n the length of the complex sequence c. the method is
more efficient when n is the product of small primes.
c a complex array of length n which contains the sequence
wsave a real work array which must be dimensioned at least 4n+15
in the program that calls pda_cfftb. the wsave array must be
initialized by calling subroutine pda_cffti(n,wsave) and a
different wsave array must be used for each different
value of n. this initialization does not have to be
repeated so long as n remains unchanged thus subsequent
transforms can be obtained faster than the first.
the same wsave array can be used by pda_cfftf and pda_cfftb.
output parameters
c for j=1,...,n
c(j)=the sum from k=1,...,n of
c(k)*exp(i*(j-1)*(k-1)*2*pi/n)
where i=sqrt(-1)
wsave contains initialization calculations which must not be
destroyed between calls of subroutine pda_cfftf or pda_cfftb
PDA [1ex