71 lines
1.7 KiB
Fortran
71 lines
1.7 KiB
Fortran
SUBROUTINE cisi(x,ci,si)
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INTEGER MAXIT
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REAL ci,si,x,EPS,EULER,PIBY2,FPMIN,TMIN
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PARAMETER (EPS=6.e-8,EULER=.57721566,MAXIT=100,PIBY2=1.5707963,
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*FPMIN=1.e-30,TMIN=2.)
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INTEGER i,k
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REAL a,err,fact,sign,sum,sumc,sums,t,term,absc
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COMPLEX h,b,c,d,del
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LOGICAL odd
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absc(h)=abs(real(h))+abs(aimag(h))
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t=abs(x)
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if(t.eq.0.)then
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si=0.
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ci=-1./FPMIN
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return
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endif
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if(t.gt.TMIN)then
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b=cmplx(1.,t)
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c=1./FPMIN
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d=1./b
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h=d
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do 11 i=2,MAXIT
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a=-(i-1)**2
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b=b+2.
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d=1./(a*d+b)
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c=b+a/c
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del=c*d
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h=h*del
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if(absc(del-1.).lt.EPS)goto 1
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11 continue
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pause 'cf failed in cisi'
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1 continue
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h=cmplx(cos(t),-sin(t))*h
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ci=-real(h)
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si=PIBY2+aimag(h)
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else
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if(t.lt.sqrt(FPMIN))then
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sumc=0.
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sums=t
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else
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sum=0.
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sums=0.
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sumc=0.
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sign=1.
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fact=1.
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odd=.true.
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do 12 k=1,MAXIT
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fact=fact*t/k
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term=fact/k
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sum=sum+sign*term
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err=term/abs(sum)
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if(odd)then
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sign=-sign
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sums=sum
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sum=sumc
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else
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sumc=sum
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sum=sums
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endif
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if(err.lt.EPS)goto 2
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odd=.not.odd
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12 continue
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pause 'maxits exceeded in cisi'
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endif
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2 si=sums
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ci=sumc+log(t)+EULER
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endif
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if(x.lt.0.)si=-si
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return
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END
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