Fix save_abc dropping the last a coefficient#27
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In run_FT, a_coeffs is appended at every iteration while b_coeffs and c_coeffs are appended only when the algorithm has not converged, so len(a) = N and len(b) = len(c) = N-1. save_abc used len(c_coeffs) as the array size, silently truncating the last diagonal element of the continued fraction. Use len(a_coeffs) as the leading dimension and fill b/c only for the rows that exist. The last row gets b = c = 0, consistent with the termination of the continued fraction. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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Summary
save_abc()usedlen(c_coeffs)as the array dimension, but in the biconjugate Lanczos (run_FT)len(a_coeffs) = Nwhilelen(b_coeffs) = len(c_coeffs) = N-1, so the last diagonal coefficienta[N-1]was silently dropped.len(a_coeffs)as the leading dimension and fillsb/conly for the available rows. The last row hasb = c = 0, consistent with the termination of the continued fraction.Test plan
save_abcnow writes all coefficients and the resulting spectrum is correct.🤖 Generated with Claude Code