• NAME
• SYNOPSIS
• F95 INTERFACE
• C INTERFACE
• ARGUMENTS

NAME

dfftb - compute a periodic sequence from its Fourier coefficients. The DFFT operations are unnormalized, so a call of DFFTF followed by a call of DFFTB will multiply the input sequence by N.

SYNOPSIS

  SUBROUTINE DFFTB( N, X, WSAVE)
  INTEGER N
  DOUBLE PRECISION X(*), WSAVE(*)

  SUBROUTINE DFFTB_64( N, X, WSAVE)
  INTEGER*8 N
  DOUBLE PRECISION X(*), WSAVE(*)

F95 INTERFACE

  SUBROUTINE FFTB( [N], X, WSAVE)
  INTEGER :: N
  REAL(8), DIMENSION(:) :: X, WSAVE

  SUBROUTINE FFTB_64( [N], X, WSAVE)
  INTEGER(8) :: N
  REAL(8), DIMENSION(:) :: X, WSAVE

C INTERFACE

#include <sunperf.h>

void dfftb(int n, double *x, double *wsave);

void dfftb_64(long n, double *x, double *wsave);

ARGUMENTS

• N (input)
  Length of the sequence to be transformed. These subroutines are most efficient when N is a product of small primes. N > = 0.

• X (input/output)
  On entry, an array of length N containing the sequence to be transformed.

• WSAVE (input)
  On entry, WSAVE must be an array of dimension (2 * N + 15) or greater and must have been initialized by DFFTI.