Hence F is the sum of an injective module and a projective module.
Here p2(r) is the sum of the squares of the divisors of r.
Every F is a sum of irreducible elements.
Here F is the sum of a collection of......
The sum is taken over all a dividing p.
[see also: add] Summing (2) and (3), we obtain......
Keep only those vertices whose coordinates sum to 4. [= add up to 4]