I am looking for the origins of efficient calculations of sums
of the form:
m m m
a(k,m) := 1 + 2 + 3 + ... + k
It seems that Gersonides (Rabbi Ben Gershon) knew
this sums for m=1,2,3.
Are there earlier instances of the "formulas" for these sums?
Do these sums have a standard name nowadays?
Thanks,
Boaz