Classical Virasoro constraint algebra
= Classical Virasoro constraint algebra
{title2=$\{L_m,L_n\}=-i(m-n)L_{m+n}$}
With the <oscillator symplectic form of an open string>, the quadratic generators $L_n=\sum_k\alpha_k\cdot\alpha_{n-k}/2$ obey $\{\alpha_k,L_n\}=-ik\alpha_{k+n}$. Their <Poisson brackets> close without a central term, so they are <first-class constraints>. Quantization requires <normal ordering> and can produce the <Virasoro central extension>.