By letting q=0, we obtain t-deformations of key polynomials and Demazure atoms and
we show that the Hall-Littlewood polynomials expand positively into these deformations.
This generalizes a result by Haglund, Luoto, Mason and van Willigenburg.
As a corollary, the Schur polynomials decompose with non-negative coefficients
into t-deformations of general Demazure atoms and
thus generalize the t=0 case which was previously known.
This gives a unified formula for the classical expansion
of Schur polynomials in Hall-Littlewood polynomials
and the expansion of Schur polynomials into Demazure atoms.
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