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de Moivre Polynomial


The name de Moivre polynomial is used for two unrelated families of polynomials, one in combinatorics and the other in the theory of algebraic equations.

In the combinatorial usage, the de Moivre polynomials are the ordinary partial Bell polynomials (O'Sullivan 2022). For nonnegative integers n and k, they are defined by the formal power series identity

 (sum_(j=1)^inftyx_jt^j)^k=sum_(n=k)^inftyA_(n,k)(x_1,x_2,...)t^n.
(1)

Thus A_(n,k) is the coefficient of t^n in the kth power of a formal power series with its t^0 coefficient equal to 0. The boundary values are A_(0,0)=1, A_(n,0)=0 for n>0, and A_(n,k)=0 for n<k. For 1<=k<=n, the relation to the exponential partial Bell polynomial is

 A_(n,k)(x_1,...,x_(n-k+1))=(k!)/(n!)B_(n,k)(1!x_1,2!x_2,...,(n-k+1)!x_(n-k+1)).
(2)

For example, A_(4,2)(x_1,x_2,x_3)=2x_1x_3+x_2^2. Multiplication of the defining formal power series gives the recurrence relation

 A_(n,k+1)=sum_(j=k)^(n-1)A_(j,k)x_(n-j),
(3)

where n>=k+1 and k>=0 (Comtet 1974, O'Sullivan 2022).

In the algebraic usage, a de Moivre polynomial is a shifted, scaled Chebyshev polynomial of the first kind. For an odd number n>=3, one normalization is

 f_n(x;d,v)=2(sqrt(v))^nT_n(x/(2sqrt(v)))-2d,
(4)

where d and v are nonzero rational numbers, and d^2-v^n has no rational square root. Here T_n(x) is the Chebyshev polynomial of the first kind. The square roots cancel, leaving rational coefficients, and the equation f_n(x;d,v)=0 can be solved using radicals (Girstmair 2022). For n=5, it becomes

 x^5-5vx^3+5v^2x-2d=0,
(5)

which has the form of de Moivre's quintic with a=-5v and b=-2d.


See also

Bell Polynomial, Chebyshev Polynomial of the First Kind, de Moivre's Quintic, Partial Bell Polynomial

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References

Comtet, L. Advanced Combinatorics: The Art of Finite and Infinite Expansions, rev. enl. ed. Dordrecht, Netherlands: Reidel, p. 133, 1974.Girstmair, K. "Chebyshev Polynomials and Galois Groups of De Moivre Polynomials." Int. J. Number Th. 18, 1735-1748, 2022. https://doi.org/10.1142/S1793042122500889.O'Sullivan, C. "De Moivre and Bell Polynomials." Expo. Math. 40, 870-893, 2022. https://doi.org/10.1016/j.exmath.2022.07.006.

Cite this as:

Weisstein, Eric W. "de Moivre Polynomial." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/deMoivrePolynomial.html

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