relibc_openlibm/slatec/cosqb.f

*DECK COSQB
      SUBROUTINE COSQB (N, X, WSAVE)
C***BEGIN PROLOGUE  COSQB
C***PURPOSE  Compute the unnormalized inverse cosine transform.
C***LIBRARY   SLATEC (FFTPACK)
C***CATEGORY  J1A3
C***TYPE      SINGLE PRECISION (COSQB-S)
C***KEYWORDS  FFTPACK, INVERSE COSINE FOURIER TRANSFORM
C***AUTHOR  Swarztrauber, P. N., (NCAR)
C***DESCRIPTION
C
C  Subroutine COSQB computes the fast Fourier transform of quarter
C  wave data. That is, COSQB computes a sequence from its
C  representation in terms of a cosine series with odd wave numbers.
C  The transform is defined below at output parameter X.
C
C  COSQB is the unnormalized inverse of COSQF since a call of COSQB
C  followed by a call of COSQF will multiply the input sequence X
C  by 4*N.
C
C  The array WSAVE which is used by subroutine COSQB must be
C  initialized by calling subroutine COSQI(N,WSAVE).
C
C
C  Input Parameters
C
C  N       the length of the array X to be transformed.  The method
C          is most efficient when N is a product of small primes.
C
C  X       an array which contains the sequence to be transformed
C
C  WSAVE   a work array which must be dimensioned at least 3*N+15
C          in the program that calls COSQB.  The WSAVE array must be
C          initialized by calling subroutine COSQI(N,WSAVE), and a
C          different WSAVE array must be used for each different
C          value of N.  This initialization does not have to be
C          repeated so long as N remains unchanged.  Thus subsequent
C          transforms can be obtained faster than the first.
C
C  Output Parameters
C
C  X       For I=1,...,N
C
C               X(I)= the sum from K=1 to K=N of
C
C                  2*X(K)*COS((2*K-1)*(I-1)*PI/(2*N))
C
C               A call of COSQB followed by a call of
C               COSQF will multiply the sequence X by 4*N.
C               Therefore COSQF is the unnormalized inverse
C               of COSQB.
C
C  WSAVE   contains initialization calculations which must not
C          be destroyed between calls of COSQB or COSQF.
C
C***REFERENCES  P. N. Swarztrauber, Vectorizing the FFTs, in Parallel
C                 Computations (G. Rodrigue, ed.), Academic Press,
C                 1982, pp. 51-83.
C***ROUTINES CALLED  COSQB1
C***REVISION HISTORY  (YYMMDD)
C   790601  DATE WRITTEN
C   830401  Modified to use SLATEC library source file format.
C   860115  Modified by Ron Boisvert to adhere to Fortran 77 by
C           (a) changing dummy array size declarations (1) to (*),
C           (b) changing definition of variable TSQRT2 by using
C               FORTRAN intrinsic function SQRT instead of a DATA
C               statement.
C   861211  REVISION DATE from Version 3.2
C   881128  Modified by Dick Valent to meet prologue standards.
C   891214  Prologue converted to Version 4.0 format.  (BAB)
C   920501  Reformatted the REFERENCES section.  (WRB)
C***END PROLOGUE  COSQB
      DIMENSION X(*), WSAVE(*)
C***FIRST EXECUTABLE STATEMENT  COSQB
      TSQRT2 = 2.*SQRT(2.)
      IF (N-2) 101,102,103
  101 X(1) = 4.*X(1)
      RETURN
  102 X1 = 4.*(X(1)+X(2))
      X(2) = TSQRT2*(X(1)-X(2))
      X(1) = X1
      RETURN
  103 CALL COSQB1 (N,X,WSAVE,WSAVE(N+1))
      RETURN
      END