\left\lbrace \;  a\; ^ {\frac{k}{d}}  \;   \right\rbrace_{k=-nd,...,nd}

a = -2 (slider from -5 to 5, step 0.1)
n = 3 (slider from 1 to 10, step 1)
d = 15 (slider from 1 to 21, step 1)
S = Sequence((k / d, a^(k / d)), k, -n * d, n * d)
b = PolyLine(S)
s = Sequence(Segment[(x(Element[S, k]), 0), Element[S, k]], k, 1, Length[S])