SUBROUTINE VCOSQB(M,N,X,XT,MDIMX,WSAVE)
C***BEGIN PROLOGUE VCOSQB
C***DATE WRITTEN 860701 (YYMMDD)
C***REVISION DATE 900509 (YYMMDD)
C***CATEGORY NO. J1A3
C***KEYWORDS FAST FOURIER TRANSFORM, COSINE TRANSFORM, ODD WAVE
C NUMBERS, MULTIPLE SEQUENCES
C***AUTHOR BOISVERT, R. F. (NIST)
C***PURPOSE Normalized inverse of VCOSQF.
C***DESCRIPTION
C
C Subroutine VCOSQB computes the backward fast Fourier cosine transform
C of M quarter wave sequences. That is, cosine series representations
C with only odd wave numbers. The transform is defined below at output
C parameter X.
C
C The array WSAVE which is used by subroutine VCOSQB must be
C initialized by calling subroutine VCOSQI(N,WSAVE).
C
C
C Input Parameters
C
C M the number of sequences to be transformed.
C
C N the length of the sequences to be transformed. The method
C is most efficient when N is a product of small primes.
C
C X an array of size at least X(MDIMX,N) which contains the
C the sequences to be transformed. The sequences are stored
C in the ROWS of X. Thus, the Jth sequence is stored in
C X(J,I), I=1,..,N.
C
C XT a work array of size at least XT(MDIMX,N).
C
C MDIMX the first dimension of the array X exactly as it appears in
C the calling program.
C
C WSAVE a work array which must be dimensioned at least 2*N+15
C in the program that calls VCOSQB. The WSAVE array must be
C initialized by calling subroutine VCOSQI(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.
C
C Output Parameters
C
C X For I=1,...,N and J=1,...,M
C
C X(I)= the sum from K=1 to K=N of
C
C 4*X(K)*COS((2*K-1)*(I-1)*PI/(2*N)) /SQRT(4*N)
C
C WSAVE contains initialization calculations which must not
C be destroyed between calls of VCOSQF or VCOSQB.
C
C -----------------------------------------------------------------
C
C NOTE - A call of VCOSQF followed immediately by a call of
C of VCOSQB will return the original sequences X. Thus,
C VCOSQB is the correctly normalized inverse VCOSQF.
C
C -----------------------------------------------------------------
C
C VCOSQB is a straightforward extension of the subprogram COSQB to
C handle M simultaneous sequences. COSQB was originally developed
C by P. N. Swarztrauber of NCAR.
C
C***REFERENCES P. N. Swarztrauber, Vectorizing the FFTs, in Parallel
C Computations, (G. Rodrigue, ed.), Academic Press, 1982,
C pp. 51-83.
C***ROUTINES CALLED (NONE)
C***END PROLOGUE VCOSQB
DIMENSION X(MDIMX,*), XT(MDIMX,*), WSAVE(*)
C***FIRST EXECUTABLE STATEMENT VCOSQB
IF (M .LE. 0) GO TO 900
IF (N .GT. 2) GO TO 300
IF (N .EQ. 2) GO TO 200
GO TO 900
C
C CASE N = 2
C
200 CONTINUE
SCALE = 2.0E0*SQRT(0.50E0)
DO 210 J=1,M
X1 = SCALE*(X(J,1)+X(J,2))
X(J,2) = X(J,1)-X(J,2)
X(J,1) = X1
210 CONTINUE
GO TO 900
C
C CASE N .GT. 2
C
C ... PREPROCESSING
C
300 CONTINUE
NS2 = (N+1)/2
NP2 = N+2
DO 310 I=3,N,2
DO 310 J=1,M
XIM1 = X(J,I-1)+X(J,I)
X(J,I) = X(J,I)-X(J,I-1)
X(J,I-1) = XIM1
310 CONTINUE
DO 320 J=1,M
X(J,1) = X(J,1)+X(J,1)
320 CONTINUE
MODN = MOD(N,2)
IF (MODN .EQ. 0) THEN
DO 330 J=1,M
X(J,N) = X(J,N)+X(J,N)
330 CONTINUE
ENDIF
C
C ... REAL, PERIODIC TRANSFORM
C
CALL VRFFTB (M,N,X,XT,MDIMX,WSAVE(N+1))
C
C ... POSTPROCESSING
C
DO 340 K=2,NS2
KC = NP2-K
DO 340 J=1,M
XT(J,K) = WSAVE(K-1)*X(J,KC)+WSAVE(KC-1)*X(J,K)
XT(J,KC) = WSAVE(K-1)*X(J,K)-WSAVE(KC-1)*X(J,KC)
340 CONTINUE
IF (MODN .EQ. 0) THEN
DO 350 J=1,M
X(J,NS2+1) = WSAVE(NS2)*(X(J,NS2+1)+X(J,NS2+1))
350 CONTINUE
ENDIF
DO 360 K=2,NS2
KC = NP2-K
DO 360 J=1,M
X(J,K) = XT(J,K)+XT(J,KC)
X(J,KC) = XT(J,K)-XT(J,KC)
360 CONTINUE
DO 370 J=1,M
X(J,1) = X(J,1)+X(J,1)
370 CONTINUE
C
C ... NORMALIZATION
C
SCALE = 0.5
DO 380 I=1,N
DO 380 J=1,M
X(J,I) = SCALE*X(J,I)
380 CONTINUE
C
C EXIT
C
900 CONTINUE
RETURN
END
.
