Language Reference |
APPCORT Call |
If is rank deficient, then the least squares problem has infinitely many solutions (Golub and Van Loan; 1989, p. 241). However, there is a unique solution which has the smallest Euclidean norm. The APPCORT subroutine computes the minimum Euclidean-norm solution of the (rank-deficient) least squares problem by applyng a complete orthogonal decomposition by Householder transformations to the vector .
The input arguments to the APPCORT subroutine are as follows:
is an matrix , with , which is to be decomposed into the product of the orthogonal matrix , the upper triangular matrix , and the orthogonal matrix ,
is a matrix, .
is an optional scalar that specifies a singularity criterion.
The APPCORT subroutine returns the following values:
is an matrix product
which is the minimum Euclidean-norm solution of the rank-deficient least squares problem .
is the number of linearly dependent columns in the matrix that are detected by applying the Householder transformations. That is, lindep is , where is the numerical rank of .
See the section COMPORT Call for information about complete orthogonal decomposition.
The following example uses the APPCORT call to solve a rank-deficient least squares problem:
/* compute solution for rank-deficient least squares problem: min |Ax-b|^2 The range of A is a line; b is a point not on the line. */ A = {1 2, 2 4, -1 -2}; b = {1, 3, -2}; call appcort(x,lindep,A,b); print x;
The argument b can also be a matrix. If b is an identity matrix, then you can use the APPCORT subroutine to form a generalized inverse, as shown in the following example:
/* A has only four linearly independent columns */ A = {1 0 1 0 0, 1 0 0 1 0, 1 0 0 0 1, 0 1 1 0 0, 0 1 0 1 0, 0 1 0 0 1 }; /* compute Moore-Penrose generalized inverse */ b = i(nrow(A)); /* identity matrix */ call appcort(Ainv, lindep, A, b); print Ainv; /* verify generalized inverse conditions (Golub & Van Loan, p. 243) */ eps = 1e-12; if any(A*Ainv*A-A > eps) | any(Ainv*A*Ainv-Ainv > eps) | any((A*Ainv)`-A*Ainv > eps) | any((Ainv*A)`-Ainv*A > eps) then msg = "Pseudoinverse conditions not satisfied"; else msg = "Pseudoinverse conditions satisfied"; print msg;
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