ComposeSeries

ComposeSeries[series1, series2, ...] composes several power series.

Examples

Compose two series:

s1 = Series[Exp[x], {x, 0, 3}];
s2 = Series[Sin[y], {y, 0, 3}];
ComposeSeries[s1, s2]

Please visit the official Wolfram Language Reference for more details.