http://mathworld.i2p/Double-FreeSet.html
An asymptotic formula for is given by (6) (Wang 1989). See also A-Sequence , Binary , Klarner-Rado Sequence , Sum-Free Set , Triple-Free Set References Finch, S. R. "Triple-Free Set Constants." §2.26 in Mathematical Constants. Cambridge, England: Cambridge University Press, pp. 183-185, 2003.