String-loop effective action expansion
= String-loop effective action expansion
{title2=$S_{\mathrm{eff}}=\sum_{g\geq0}g_s^{2g-2}S_g$}
Connected oriented <closed string> worldsheets of genus $g$ give the spacetime effective-action term $g_s^{2g-2}S_g$. Each $S_g$ has a separate low-energy derivative expansion in $\alpha'$. The <string coupling> orders quantum string loops, while $\alpha'$ orders effects of finite string size.