subroutine spofa(a,lda,n,info) integer lda,n,info real a(lda,1) c c spofa factors a real symmetric positive definite matrix. c c spofa is usually called by spoco, but it can be called c directly with a saving in time if rcond is not needed. c (time for spoco) = (1 + 18/n)*(time for spofa) . c c on entry c c a real(lda, n) c the symmetric matrix to be factored. only the c diagonal and upper triangle are used. c c lda integer c the leading dimension of the array a . c c n integer c the order of the matrix a . c c on return c c a an upper triangular matrix r so that a = trans(r)*r c where trans(r) is the transpose. c the strict lower triangle is unaltered. c if info .ne. 0 , the factorization is not complete. c c info integer c = 0 for normal return. c = k signals an error condition. the leading minor c of order k is not positive definite. 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 sdot c fortran sqrt c c internal variables c real sdot,t real s integer j,jm1,k c begin block with ...exits to 40 c c do 30 j = 1, n info = j s = 0.0e0 jm1 = j - 1 if (jm1 .lt. 1) go to 20 do 10 k = 1, jm1 t = a(k,j) - sdot(k-1,a(1,k),1,a(1,j),1) t = t/a(k,k) a(k,j) = t s = s + t*t 10 continue 20 continue s = a(j,j) - s c ......exit if (s .le. 0.0e0) go to 40 a(j,j) = sqrt(s) 30 continue info = 0 40 continue return end