For a closed form I'm betting?
g(0)=-12
g(1)=g(0)+1=-11
g(2)=g(1)+2=-9
g(3)=g(2)+3=-6
...
The common difference is the input.
g(x) = -12+x(x+1)/2
This is easy to prove for INTEGER values, and I'm suspecting that is what is being asked. If not, report back.
