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71f2260186
When eigenvalues are nearly degenerate, numerical accumulation in the divide-and-conquer merge can push w[i] slightly above zero. The subsequent sqrt(-w[i]) then produces NaN, which propagates through the entire eigenvector output of DSYEVD/DSTEDC. Use fmax(-w[i], 0.0) to safely clamp the sqrt argument, yielding zero instead of NaN for spuriously positive w[i]. The fmax call maps to a single vmaxsd instruction on x86_64 with zero overhead. Add test/test_laed3_nan.c which exercises DSYEVD on a rank-1 perturbed identity matrix designed to create near-degenerate eigenvalue clusters. |
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| Makefile | ||
| laed3_parallel.c | ||
| laed3_single.c | ||