Is it a sum-free set?
Python 2, 41 bytes
lambda s:s==s-{a+b for a in s for b in s}
s
should be a Python set.
Fun fact: sum-free
is an anagram of my name.
Jelly, 5 bytes
ṗ3ḅ-P
Try it online!
How it works
ṗ3ḅ-P Main link. Argument: A (array)
ṗ3 Take the third Cartesian power of A, i.e., generate all triplets that
consist of elements of A.
ḅ- Convert each triplet from base -1 to integer.
This maps [a, b, c] to a - b + c = (a + c) - b.
If (a + c) belong to A, this will yield 0 for some b.
P Take the product of all resulting integers.
Pyth - 8 5 bytes
Thanks to @FryAmTheEggman for saving me 3 bytes.
!@sM*
Test Suite.
! Logical not. This makes the empty intersection true and vice versa.
@ Q Setwise intersection with input (implictly).
sM Map sum to all the pairs.
*QQ Get all pairs by doing cartesian product with input*input (implicit).