Notations that are mnemonic outside of English Some conventional math notations seem arbitrary to English speakers but were mnemonic to non-English speakers who started them. To give a simple example, Z is the symbol for integers because of the German word _Zahl(en)_ 'number(s)'. What are some more examples?

In topology the letter $F$ is commonly used to denote a closed set, from French _fermé_ 'closed [set]'. The common use of $K$ to denote a compact set is probably from German _kompakt_ , as in _kompakte Menge_ 'compact set' and _kompakter Raum_ 'compact space'. The common use of $k$ to denote an arbitrary field is probably from German _Körper_ 'field'. The common use of $G$ for an open set is probably from German _Gebiet_ 'region', though as a mathematical term it now means 'non-empty, connected, open set'. The notation $G_\delta$- _set_ for the intersection of countably many open sets combines this $G$ with $\delta$ for German _Durchschnitt_ 'intersection'. Presumably $F_\sigma$- _set_ for the union of countably many closed sets is from the $F$ above and $\sigma$ for French _somme_ 'sum'. The $T$ in the names of the separation axioms $T_1,T_2$, etc. is from German _Trennungsaxiom_ 'separation axiom'.