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?

**Answer**

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 Gδ–*set* for the intersection of countably many open sets combines this G with δ for German *Durchschnitt* ‘intersection’. Presumably Fσ–*set* for the union of countably many closed sets is from the F above and σ for French *somme* ‘sum’. The T in the names of the separation axioms T1,T2, etc. is from German *Trennungsaxiom* ‘separation axiom’.

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