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fitpack/fpcyt2.f

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+      recursive subroutine fpcyt2(a,n,b,c,nn)
+      implicit none
+c subroutine fpcyt2 solves a linear n x n system
+c         a * c = b
+c where matrix a is a cyclic tridiagonal matrix, decomposed
+c using subroutine fpsyt1.
+c  ..
+c  ..scalar arguments..
+      integer n,nn
+c  ..array arguments..
+      real*8 a(nn,6),b(n),c(n)
+c  ..local scalars..
+      real*8 cc,sum
+      integer i,j,j1,n1
+c  ..
+      c(1) = b(1)*a(1,4)
+      sum = c(1)*a(1,5)
+      n1 = n-1
+      do 10 i=2,n1
+         c(i) = (b(i)-a(i,1)*c(i-1))*a(i,4)
+         sum = sum+c(i)*a(i,5)
+  10  continue
+      cc = (b(n)-sum)*a(n,4)
+      c(n) = cc
+      c(n1) = c(n1)-cc*a(n1,6)
+      j = n1
+      do 20 i=3,n
+         j1 = j-1
+         c(j1) = c(j1)-c(j)*a(j1,3)*a(j1,4)-cc*a(j1,6)
+         j = j1
+  20  continue
+      return
+      end