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 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’.