1 files changed, 90 insertions, 0 deletions
diff --git a/math/ieee/chap1/iftsym.f b/math/ieee/chap1/iftsym.f
new file mode 100644
index 00000000..e45c9a37
--- /dev/null
+++ b/math/ieee/chap1/iftsym.f
@@ -0,0 +1,90 @@
+c
+c-----------------------------------------------------------------------
+c subroutine: iftsym
+c compute idft for real, symmetric, n-point sequence x(m) using
+c n/2-point fft
+c symmetric sequence means x(m)=x(n-m), m=1,...,n/2-1
+c note: index m is sequence index--not fortran index
+c-----------------------------------------------------------------------
+c
+      subroutine iftsym(x, n, y)
+      dimension x(1), y(1)
+c
+c x = real array which on input contains the n/2+1 real points of the
+c     transform of the input--i.e. the zero valued imaginary parts
+c     are not given as input
+c     on output x contains the n/2+1 points of the time sequence
+c     (symmetrical)
+c n = true size of input
+c y = scratch array of size n/2+2
+c
+c
+c for n = 2, compute idft directly
+c
+      if (n.gt.2) go to 10
+      t = (x(1)+x(2))/2.
+      x(2) = (x(1)-x(2))/2.
+      x(1) = t
+      return
+  10  twopi = 8.*atan(1.0)
+c
+c first compute x1=x(1) term directly
+c use recursion on the sine cosine terms
+c
+      no2 = n/2
+      no4 = n/4
+      tpn = twopi/float(n)
+      cosd = cos(tpn)
+      sind = sin(tpn)
+      cosi = 2.
+      sini = 0.
+      x1 = x(1) - x(no2+1)
+      do 20 i=2,no2
+        temp = cosi*cosd - sini*sind
+        sini = cosi*sind + sini*cosd
+        cosi = temp
+        x1 = x1 + x(i)*cosi
+  20  continue
+      x1 = x1/float(n)
+c
+c scramble original dft (x(k)) to give y(k)
+c use recursion relation to generate sin(tpn*i) multiplier
+c
+      cosi = cos(tpn)
+      sini = sin(tpn)
+      cosd = cosi
+      sind = sini
+      y(1) = (x(1)+x(no2+1))/2.
+      y(2) = 0.
+      nind = no4 + 1
+      do 30 i=2,nind
+        ind = 2*i
+        nind1 = no2 + 2 - i
+        ak = (x(i)+x(nind1))/2.
+        bk = (x(i)-x(nind1))
+        y(ind-1) = ak
+        y(ind) = bk*sini
+        temp = cosi*cosd - sini*sind
+        sini = cosi*sind + sini*cosd
+        cosi = temp
+  30  continue
+c
+c take n/2 point idft of y
+c
+      call fsst(y, no2)
+c
+c form x sequence from y sequence
+c
+      x(1) = y(1)
+      x(2) = x1
+      if (n.eq.4) go to 50
+      do 40 i=2,no4
+        ind = 2*i
+        ind1 = no2 + 2 - i
+        x(ind-1) = (y(i)+y(ind1))/2.
+        t1 = (y(i)-y(ind1))/2.
+        x(ind) = t1 + x(ind-2)
+  40  continue
+  50  x(no2+1) = y(no4+1)
+      return
+      end