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Lib/interpolate/fitpack/fpcyt2.f

 

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      subroutine fpcyt2(a,n,b,c,nn)

 

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c subroutine fpcyt2 solves a linear n x n system

 

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c         a * c = b

 

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c where matrix a is a cyclic tridiagonal matrix, decomposed

 

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c using subroutine fpsyt1.

 

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c  ..

 

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c  ..scalar arguments..

 

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      integer n,nn

 

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c  ..array arguments..

 

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      real*8 a(nn,6),b(n),c(n)

 

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c  ..local scalars..

 

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      real*8 cc,sum

 

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      integer i,j,j1,n1

 

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c  ..

 

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      c(1) = b(1)*a(1,4)

 

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      sum = c(1)*a(1,5)

 

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      n1 = n-1

 

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      do 10 i=2,n1

 

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         c(i) = (b(i)-a(i,1)*c(i-1))*a(i,4)

 

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         sum = sum+c(i)*a(i,5)

 

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  10  continue

 

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      cc = (b(n)-sum)*a(n,4)

 

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      c(n) = cc

 

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      c(n1) = c(n1)-cc*a(n1,6)

 

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      j = n1

 

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      do 20 i=3,n

 

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         j1 = j-1

 

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         c(j1) = c(j1)-c(j)*a(j1,3)*a(j1,4)-cc*a(j1,6)

 

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         j = j1

 

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  20  continue

 

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      return

 

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      end

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