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The Stanley-Stembridge Conjecture for $\bf 2 + 1 +1$-avoiding unit interval orders
A natural unit interval order is a naturally labelled partially ordered set that avoids patterns ${\bf 3 + \bf 1}$ and $\bf 2 + 2$. To each natural unit interval order one can associate a symmetric function. The Stanley-Stembridge conjecture states that each such symmetric function is positive in the …