Given a directed graph $G$, how to find a subgraph $G'$, where every node in $G'$ is isolated and the size of $G'$ is largest possible ?
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closed as offtopic by Max Alekseyev, user44191, Chris Godsil, Wolfgang, darij grinberg May 15 at 7:22
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4$\begingroup$ Isn't that the independence number problem? $\endgroup$ – Bullet51 May 11 at 3:11