[Help-glpk] Sign of an integer

[hamsda9]

Hello,

i am searching for an expression equal to the mathematicl sign function.
I have a sum in a "subject to" statement, and i want the returning value of
this sum to be zero if the sum is 0 or to be one if the sum is greater than
0.

Example:
subject to rooms{a in A}: sum{b in B} sign(sum{c in C} x[a,b,c]) <= r;
whereas x is a variable only containing binary values.

any ideas?

Thanks in advance
Mike H.
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