Question 19
A positive integer $N$ is called “visible” if $N=\lfloor x\rfloor+\lfloor2x\rfloor+\lfloor3x\rfloor$ for some real $x$; otherwise it is “invisible”. For example, $8=\lfloor1.5\rfloor+\lfloor2(1.5)\rfloor+\lfloor3(1.5)\rfloor$ is visible, while $10$ is invisible. Find the $2011$th invisible positive integer in increasing order.