  It was recently conjectured that the AGT correspondence between the
$U(r)$-instanton counting on $\mathbb R^4/\mathbb Z_p$ and the two-dimensional
field theories with the conformal symmetry algebra $\mathcal A(r,p)$ can be
considered as a root of unity limit of its K-theoretic analogue. From this
point of view, the algebra $\mathcal A(r,p)$ and a special basis in its
representation are limits of the Ding-Iohara algebra and the Macdonald
polynomials respectively. In this paper we confirm this conjecture for the
special case $r=1$. We uncover the implicit $\mathcal A(1,p)$ symmetry in this
limit. We also found that the vertex operators in the special basis have
factorized AFLT form.
