The paper 'C.C. Paige, Z.
Strakoš: Core problems in linear algebraic systems (SIAM J. Matrix Anal.
Appl. 27 (2006), pp. 861-875)' presents a core problem formulation of orthogonally invariant linear approximation problems Ax ~ b, that extracts the necessary and sufficient information for solving the original problem. Our contribution derives the fundamental characteristics of the core problem from the known relationship between the Golub-Kahan bidiagonalization and the Lanczos tridiagonalization.