Deciding D-stability is coNP-hard

The computational difficulty of D-stability, raised by Chen, Fan and Yu (1995, doi:10.1016/0167-6911(94)00036-U), is addressed by an explicit integer-matrix reduction from PARTITION. For positive weights of total 2T, the construction produces a matrix of order m + 4 that is D-stable exactly when no subset has weight T. When an equal split exists, some positive diagonal scaling has an eigenvalue with strictly positive real part. A Pell recursion supplies the parameters, with a logarithmic search bound and an explicit bound on every matrix entry. These bounds give polynomial binary output length. Together with the classical NP-completeness of PARTITION, the reduction yields coNP-hardness of D-stability and D-semistability, and NP-hardness of strict D-instability, even for a four-dimensional core with attachments at one coordinate. The hardness is in the weak sense for binary encoding. Running-time analysis and the trivial-instance cases are treated conventionally; registered Lean statements verify the explicit arithmetic encoding, bounds and promise dichotomy.