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57 lines
1.9 KiB
Fortran
57 lines
1.9 KiB
Fortran
*DECK COSQB1
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SUBROUTINE COSQB1 (N, X, W, XH)
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C***BEGIN PROLOGUE COSQB1
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C***SUBSIDIARY
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C***PURPOSE Compute the unnormalized inverse of COSQF1.
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C***LIBRARY SLATEC (FFTPACK)
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C***CATEGORY J1A3
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C***TYPE SINGLE PRECISION (COSQB1-S)
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C***KEYWORDS FFTPACK, FOURIER TRANSFORM
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C***AUTHOR Swarztrauber, P. N., (NCAR)
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C***DESCRIPTION
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C
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C Subroutine COSQB1 computes the fast Fourier transform of quarter
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C wave data. That is, COSQB1 computes a sequence from its
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C representation in terms of a cosine series with odd wave numbers.
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C The transform is defined below at output parameter X.
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C
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C***REFERENCES P. N. Swarztrauber, Vectorizing the FFTs, in Parallel
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C Computations (G. Rodrigue, ed.), Academic Press,
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C 1982, pp. 51-83.
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C***ROUTINES CALLED RFFTB
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C***REVISION HISTORY (YYMMDD)
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C 790601 DATE WRITTEN
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C 830401 Modified to use SLATEC library source file format.
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C 860115 Modified by Ron Boisvert to adhere to Fortran 77 by
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C changing dummy array size declarations (1) to (*).
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C 881128 Modified by Dick Valent to meet prologue standards.
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C 891214 Prologue converted to Version 4.0 format. (BAB)
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C 920501 Reformatted the REFERENCES section. (WRB)
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C***END PROLOGUE COSQB1
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DIMENSION X(*), W(*), XH(*)
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C***FIRST EXECUTABLE STATEMENT COSQB1
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NS2 = (N+1)/2
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NP2 = N+2
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DO 101 I=3,N,2
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XIM1 = X(I-1)+X(I)
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X(I) = X(I)-X(I-1)
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X(I-1) = XIM1
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101 CONTINUE
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X(1) = X(1)+X(1)
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MODN = MOD(N,2)
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IF (MODN .EQ. 0) X(N) = X(N)+X(N)
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CALL RFFTB (N,X,XH)
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DO 102 K=2,NS2
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KC = NP2-K
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XH(K) = W(K-1)*X(KC)+W(KC-1)*X(K)
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XH(KC) = W(K-1)*X(K)-W(KC-1)*X(KC)
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102 CONTINUE
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IF (MODN .EQ. 0) X(NS2+1) = W(NS2)*(X(NS2+1)+X(NS2+1))
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DO 103 K=2,NS2
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KC = NP2-K
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X(K) = XH(K)+XH(KC)
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X(KC) = XH(K)-XH(KC)
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103 CONTINUE
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X(1) = X(1)+X(1)
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RETURN
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END
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