For saying that x is proportional to y, one would write: $$x \propto y$$
But how could I write it if x isn't proportional to y?
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$\begingroup$This is more a question for tex stackexchange. The command "propto" can be negated by prescribing "not" to it: $$ x\propto y, \quad x\not\propto y. $$ Better ideas are given here.
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