default(realprecision, 80);
bestappr(lfun(lfuncreate(x^3 - x^2 - 2*x + 1), -1), 10^6) \\ -1/21, for D = 49Each value is computed in exact rational arithmetic from Siegel's formula (1). The generator enumerates the 173 totally real cubic fields with $D\leq 5000$ by Hunter's theorem [5], using the same polredabs ordering as the cubic regulator and residue tables, and refuses the run unless the count is 173.
The Siegel coefficients are checked in exact arithmetic against the cyclic formulas for $\mathbb{Q}(\zeta_7)^+$ and $\mathbb{Q}(\zeta_9)^+$, using generalized Bernoulli numbers for cubic Dirichlet characters. Every returned value is also compared with PARI's lfun value at 120 digits, recognised as a rational with denominator at most $10^{20}$, and the generator refuses a value if the two computations disagree or if the sign is not $(-1)^m$ at $s=1-2m$. The class number in each entry comment is computed by PARI's bnfinit and certified by bnfcertify.