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subroutine fpcyt1(a,n,nn)
c (l u)-decomposition of a cyclic tridiagonal matrix with the non-zero
c elements stored as follows
c
c | a(1,2) a(1,3) a(1,1) |
c | a(2,1) a(2,2) a(2,3) |
c | a(3,1) a(3,2) a(3,3) |
c | ............... |
c | a(n-1,1) a(n-1,2) a(n-1,3) |
c | a(n,3) a(n,1) a(n,2) |
c
c ..
c ..scalar arguments..
integer n,nn
c ..array arguments..
real*8 a(nn,6)
c ..local scalars..
real*8 aa,beta,gamma,sum,teta,v,one
integer i,n1,n2
c ..
c set constant
one = 1
n2 = n-2
beta = one/a(1,2)
gamma = a(n,3)
teta = a(1,1)*beta
a(1,4) = beta
a(1,5) = gamma
a(1,6) = teta
sum = gamma*teta
do 10 i=2,n2
v = a(i-1,3)*beta
aa = a(i,1)
beta = one/(a(i,2)-aa*v)
gamma = -gamma*v
teta = -teta*aa*beta
a(i,4) = beta
a(i,5) = gamma
a(i,6) = teta
sum = sum+gamma*teta
10 continue
n1 = n-1
v = a(n2,3)*beta
aa = a(n1,1)
beta = one/(a(n1,2)-aa*v)
gamma = a(n,1)-gamma*v
teta = (a(n1,3)-teta*aa)*beta
a(n1,4) = beta
a(n1,5) = gamma
a(n1,6) = teta
a(n,4) = one/(a(n,2)-(sum+gamma*teta))
return
end
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