%I #24 Apr 28 2019 11:03:10
%S 0,0,1,1,1,1,2,2,2,3,3,4,4,4,4,5,5,5,6,6,7,7,7,7,8,8,9,9,9,9,10,10,11,
%T 11,11,11,12,12,13,13,13,13,13,14,14,15,15,15,16,16,16,17,17,18,18,18,
%U 18,19,19,19,19,20,21
%N Size of the smallest set hitting all {x, 2x, 3x} contained in D(n) = the first n 3-smooth numbers {1, 2, 3, 4, 6, 8, 9, 12, 16, 18, 24, 27,...} (A003586).
%C A057561(n) = n - a(n). [_Steven Finch_, Feb 25 2009]
%H F. Chung, P. Erdős and R. L. Graham, <a href="http://www.math.ucsd.edu/~ronspubs/02_04_linear_forms.pdf">On Sparse Sets Hitting Linear Forms</a>, in Number Theory for the Millennium I (M. A. Bennett et al., eds.), 2002, pp. 257-272.
%H Steven R. Finch, <a href="/FinchTriple.html">Triple-Free Sets of Integers</a> [From Steven Finch, Apr 20 2019]
%Y Cf. A003586, A057561.
%K nonn
%O 1,7
%A _Barry Cipra_, Jun 15 2004
%E More terms from _Sean A. Irvine_, Nov 19 2015