| subroutine dpbsl(abd,lda,n,m,b) |
| |
| integer lda,n,m |
| double precision abd(lda,n),b(n) |
| c |
| c dpbsl solves the double precision symmetric positive definite |
| c band system a*x = b |
| c using the factors computed by dpbco or dpbfa. |
| c |
| c on entry |
| c |
| c abd double precision(lda, n) |
| c the output from dpbco or dpbfa. |
| c |
| c lda integer |
| c the leading dimension of the array abd . |
| c |
| c n integer |
| c the order of the matrix a . |
| c |
| c m integer |
| c the number of diagonals above the main diagonal. |
| c |
| c b double precision(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 dpbco(abd,lda,n,rcond,z,info) |
| c if (rcond is too small .or. info .ne. 0) go to ... |
| c do 10 j = 1, p |
| c call dpbsl(abd,lda,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 daxpy,ddot |
| c fortran min0 |
| c |
| c internal variables |
| c |
| double precision ddot,t |
| integer k,kb,la,lb,lm |
| c |
| c solve trans(r)*y = b |
| c |
| do 10 k = 1, n |
| lm = min0(k-1,m) |
| la = m + 1 - lm |
| lb = k - lm |
| t = ddot(lm,abd(la,k),1,b(lb),1) |
| b(k) = (b(k) - t)/abd(m+1,k) |
| 10 continue |
| c |
| c solve r*x = y |
| c |
| do 20 kb = 1, n |
| k = n + 1 - kb |
| lm = min0(k-1,m) |
| la = m + 1 - lm |
| lb = k - lm |
| b(k) = b(k)/abd(m+1,k) |
| t = -b(k) |
| call daxpy(lm,t,abd(la,k),1,b(lb),1) |
| 20 continue |
| return |
| end |
| |
