SUBROUTINE COSQB1 (N,X,W,XH)
      IMPLICIT DOUBLE PRECISION (A-H, O-Z)
      DIMENSION       X(1)       ,W(1)       ,XH(1)
      NS2 = (N+1)/2
      NP2 = N+2
      DO 101 I=3,N,2
         XIM1 = X(I-1)+X(I)
         X(I) = X(I)-X(I-1)
         X(I-1) = XIM1
  101 CONTINUE
      X(1) = X(1)+X(1)
      MODN = MOD(N,2)
      IF (MODN .EQ. 0) X(N) = X(N)+X(N)
      CALL RFFTB (N,X,XH)
      DO 102 K=2,NS2
         KC = NP2-K
         XH(K) = W(K-1)*X(KC)+W(KC-1)*X(K)
         XH(KC) = W(K-1)*X(K)-W(KC-1)*X(KC)
  102 CONTINUE
      IF (MODN .EQ. 0) X(NS2+1) = W(NS2)*(X(NS2+1)+X(NS2+1))
      DO 103 K=2,NS2
         KC = NP2-K
         X(K) = XH(K)+XH(KC)
         X(KC) = XH(K)-XH(KC)
  103 CONTINUE
      X(1) = X(1)+X(1)
      RETURN
      END