subroutine sppsl(ap,n,b)
integer n
real ap(1),b(1)
c
c sppsl solves the real symmetric positive definite system
c a * x = b
c using the factors computed by sppco or sppfa.
c
c on entry
c
c ap real (n*(n+1)/2)
c the output from sppco or sppfa.
c
c n integer
c the order of the matrix a .
c
c b real(n)
c the right hand side vector.
c
c on return
c
c b the solution vector x .
c
c error condition
c
c a division by zero will occur if the input factor contains
c a zero on the diagonal. technically this indicates
c singularity but it is usually caused by improper subroutine
c arguments. it will not occur if the subroutines are called
c correctly and info .eq. 0 .
c
c to compute inverse(a) * c where c is a matrix
c with p columns
c call sppco(ap,n,rcond,z,info)
c if (rcond is too small .or. info .ne. 0) go to ...
c do 10 j = 1, p
c call sppsl(ap,n,c(1,j))
c 10 continue
c
c linpack. this version dated 08/14/78 .
c cleve moler, university of new mexico, argonne national lab.
c
c subroutines and functions
c
c blas saxpy,sdot
c
c internal variables
c
real sdot,t
integer k,kb,kk
c
kk = 0
do 10 k = 1, n
t = sdot(k-1,ap(kk+1),1,b(1),1)
kk = kk + k
b(k) = (b(k) - t)/ap(kk)
10 continue
do 20 kb = 1, n
k = n + 1 - kb
b(k) = b(k)/ap(kk)
kk = kk - k
t = -b(k)
call saxpy(k-1,t,ap(kk+1),1,b(1),1)
20 continue
return
end