Hosack, J M.(1970). The spectrum of a selfadjoint compression of a self-adjoint operator. Proceedings of the American Mathematical Society, 25(1), 83-83. doi:10.1090/s0002-9939-1970-0271760-9

Hosack, John M. "The Spectrum of a Selfadjoint Compression of a Self-adjoint Operator". Proceedings of the American Mathematical Society, 25.1 ( 1970 ): 83-83.