Arrowhead matrix

In the mathematical field of linear algebra, an arrowhead matrix is a square matrix containing zeros in all entries except for the first row, first column, and main diagonal, these entries can be any number. In other words, the matrix has the form
Any symmetric permutation of the arrowhead matrix, formula_2, where "P" is a permutation matrix, is a (permuted) arrowhead matrix. Real symmetric arrowhead matrices are used in some algorithms for finding of eigenvalues and eigenvectors.
Real symmetric arrowhead matrices.
Let "A" be a real symmetric (permuted) arrowhead matrix of the form
where formula_4 is diagonal matrix of order "n-1",
formula_5 is a vector and formula_6 is a scalar. Let
be the eigenvalue decomposition of "A", where 
formula_8
is a diagonal matrix whose diagonal elements are the eigenvalues of "A", and
formula_9
is an orthonormal matrix whose columns are the corresponding eigenvectors. The following holds:
Symmetric arrowhead matrices arise in descriptions of radiationless transitions in isolated molecules and oscillators vibrationally coupled with a Fermi liquid.
Eigenvalues and eigenvectors.
A symmetric arrowhead matrix is irreducible if formula_13 for all "i" and formula_24 for all formula_19. The eigenvalues of an irreducible real symmetric arrowhead matrix are the zeros of the secular equation
which can be, for example, computed by the bisection method. The corresponding eigenvectors are equal to 
Direct application of the above formula may yield eigenvectors which are not numerically sufficiently orthogonal. 
The forward stable algorithm which computes each eigenvalue and each component of the corresponding eigenvector to almost full accuracy is described in. The Julia version of the software is available.
Inverses.
Let "A" be an irreducible real symmetric arrowhead matrix. If formula_28 for some "i", the inverse is a permuted irreducible real symmetric arrowhead matrix:
where
If formula_31 for all "i", the inverse is a "rank-one modification of a diagonal matrix" (diagonal-plus-rank-one matrix or DPR1):
where