Rank-preserving extensions of band matrices
Robert Leslie Ellis, David C. Lay · Linear and Multilinear Algebra · 1990
Existence and uniqueness problems are studied for extensions of a band matrix R such that the rank of the extension does not exceed the maximum of the ranks of the submatrices in the band of R. Applications are given to positive semidefinite extensions and extensions of lower-triangular matrices to contractions or to unitary matrices.