 Half-integer open-string oscillator

For a Neumann-Dirichlet open-string boundary condition, the transverse modes have frequencies $r\in \mathbb{Z}+1/2$. The canonical commutation relations give $[{\alpha }_r^i,{\alpha }_s^j]=r{\delta }^{ij}{\delta }_{r+s,0}$ and ${\alpha }_r^{†}={\alpha }_{-r}$. One derivation expands $X-y={\sum }_{r>0}{q}_r\cos (r\sigma )$: spatial orthogonality reduces the Polyakov action to $(8{\alpha }^{\prime }{)}^{-1}\int {\sum }_r({\dot{q}}_r^2-r^2{q}_r^2)d\tau$. These are harmonic oscillators of mass $1/(4{\alpha }^{\prime })$, whose normalized creation operators satisfy ${\alpha }_{-r}=i\sqrt{r}{a}_r^{†}$ in the conventional phased expansion.