[Bug-gsl] solve_symm_tridiag not valid for general symmetric tridiagonal systems

[Duane Nykamp]

The function gsl_linalg_solve_symm_tridiag is documented (and named) as
a solver for a general symmetric tridiagonal system.  It uses a variant
of Cholesky decomposition, treating the diagonal differently so that the 
algorithm applies to more than just positive definite matrices.

However, if that diagonal in this decomposition is ever zero (e.g., the 
upper-left entry of the original matrix is zero), the algorithm will 
divide by zero without checking, producing NaNs.

Duane Nykamp


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