subroutine drfti1 (n,wa,ifac) double precision wa(1), arg, argh, argld, fi, tpi integer ifac(1), ntryh(4) data ntryh(1), ntryh(2), ntryh(3), ntryh(4) /4, 2, 3, 5/ data tpi / 6.2831853071 7958647692 5286766559 00577d0/ c nl = n nf = 0 j = 0 c 101 j = j+1 if (j.le.4) ntry = ntryh(j) if (j.gt.4) ntry = ntry + 2 104 nq = nl/ntry nr = nl-ntry*nq if (nr.ne.0) go to 101 c 105 nf = nf+1 ifac(nf+2) = ntry nl = nq if (ntry .ne. 2) go to 107 if (nf .eq. 1) go to 107 do 106 i=2,nf ib = nf-i+2 ifac(ib+2) = ifac(ib+1) 106 continue ifac(3) = 2 107 if (nl .ne. 1) go to 104 ifac(1) = n ifac(2) = nf c argh = tpi/dfloat(n) is = 0 nfm1 = nf-1 l1 = 1 if (nfm1 .eq. 0) return do 110 k1=1,nfm1 ip = ifac(k1+2) ld = 0 l2 = l1*ip ido = n/l2 ipm = ip-1 do 109 j=1,ipm ld = ld+l1 i = is argld = dfloat(ld)*argh fi = 0.d0 do 108 ii=3,ido,2 i = i+2 fi = fi+1.d0 arg = fi*argld wa(i-1) = dcos(arg) wa(i) = dsin(arg) 108 continue is = is+ido 109 continue c l1 = l2 110 continue c return end