= Bosonic-string critical dimension from ghost descendants
{title2=$L_2L_{-2}|\Omega\rangle=(2\alpha_0^2+D/2-17)|\Omega\rangle$}
For the <FP ghost> vacuum annihilated by $b_{n\ge0}$ and $c_{n>0}$, the <FP ghost> level-one and level-two descendant coefficients are $-2$ and $-17$. Matter contributes $\alpha_0^2$ and $2\alpha_0^2+D/2$. <Virasoro closure from BRST nilpotence> at level one fixes the total highest weight to $\alpha_0^2/2-1$; level two then requires $D=26$. Imposing ground-state BRST closure gives $\alpha_0^2=2$ and the open bosonic <tachyon>. Interpret the coefficients with the stated <ghost zero-mode pairing>.
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