We find that algebraic sums of decay rates are fairly common, which make a notation such as $\gamma_{\widetilde{ij}}=i \gamma_a + j\gamma_\sigma$ useful (e.g., $\gamma_{\widetilde{31}}=3\gamma_a+\gamma_\sigma$). This is, however, a bit of a pain to typeset:

(e.g., $\gamma_{\widetilde{31}}=3\gamma_a+\gamma_\sigma$)

In emacs, this regexp substitution allows to transliterate \gamma_{31} into the widetilde form:

Replace regexp (default _{\([0-9][0-9]\)} -> _{\\widetilde{\1}}):