If is true precisely unless , then implies not- and not- implies . J. H. Conway has suggested the term "unlesss" for this state of affairs, by analogy with iff.
See alsoAt Least One, Exactly One, Iff, Unless
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Cite this as:
Weisstein, Eric W. "Precisely Unless." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/PreciselyUnless.html