Abel polynomials

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The Abel polynomials are a sequence of polynomials named after Niels Henrik Abel, defined by the following equation:

Contents

They occur, for instance, in the enumeration of trees (specifically, rooted labeled forests) as well as in connection with geometric probability (the random placement of nonoverlapping arcs on a circle). This polynomial sequence is of binomial type: conversely, every polynomial sequence of binomial type may be obtained from the Abel sequence using umbral calculus.

Examples

For a = 1, the polynomials are (sequence A137452 in the OEIS )

For a = 2, the polynomials are

References