2015-04-22 13:48:03 -04:00
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subroutine polyfit(x,y,sigmay,npts,nterms,mode,a,chisqr)
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2014-01-08 13:38:15 -05:00
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implicit real*8 (a-h,o-z)
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real*8 x(npts), y(npts), sigmay(npts), a(nterms)
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real*8 sumx(19), sumy(10), array(10,10)
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! Accumulate weighted sums
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nmax = 2*nterms-1
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sumx=0.
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sumy=0.
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chisq=0.
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do i=1,npts
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xi=x(i)
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yi=y(i)
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if(mode.lt.0) then
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weight=1./abs(yi)
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else if(mode.eq.0) then
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weight=1
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else
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weight=1./sigmay(i)**2
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end if
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xterm=weight
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do n=1,nmax
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sumx(n)=sumx(n)+xterm
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xterm=xterm*xi
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enddo
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yterm=weight*yi
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do n=1,nterms
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sumy(n)=sumy(n)+yterm
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yterm=yterm*xi
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enddo
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chisq=chisq+weight*yi**2
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enddo
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! Construct matrices and calculate coefficients
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do j=1,nterms
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do k=1,nterms
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n=j+k-1
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array(j,k)=sumx(n)
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enddo
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enddo
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delta=determ(array,nterms)
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if(delta.eq.0) then
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chisqr=0.
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a=0.
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else
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do l=1,nterms
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do j=1,nterms
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do k=1,nterms
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n=j+k-1
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array(j,k)=sumx(n)
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enddo
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array(j,l)=sumy(j)
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enddo
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a(l)=determ(array,nterms)/delta
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enddo
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! Calculate chi square
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do j=1,nterms
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chisq=chisq-2*a(j)*sumy(j)
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do k=1,nterms
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n=j+k-1
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chisq=chisq+a(j)*a(k)*sumx(n)
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enddo
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enddo
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free=npts-nterms
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chisqr=chisq/free
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end if
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return
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2015-04-22 13:48:03 -04:00
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end subroutine polyfit
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