invertible formal power series
Theorem. Let R be a commutative ring with non-zero unity. A formal power series
f(X):=∞∑i=0aiXi | (1) |
is invertible in the ring R[[X]] iff a0 is invertible in the ring R.
Proof. 1∘. Let f(X) have the multiplicative inverse g(X):=∑∞i=0biXi. Since
f(X)g(X)=∞∑i=0i∑j=0ajbi-jXi= 1, |
we see that a0b0=1, i.e. a0 is an invertible element (unit) of R.
2∘. Assume conversely that a0 is invertible in R. For making from a formal power series
g(X):=∞∑i=0biXi | (2) |
the inverse of f(X)=∑∞i=0aiXi, we first choose b0:=a-10. For all already defined coefficients b0,b1,…,bi-1 let the next coefficient be defined as
bi:=-a-10(a1bi-1+a2bi-2+…+aib0). |
This equation means that
i∑j=0ajbi-j=a0bi+a1bi-1+a2bi-2+…+aib0 |
vanishes for all i=1, 2,…; since a0b0=1, the product of the formal power series (1) and (2) becomes simply equal to 1. Accordingly, f(x) is invertible.
Title | invertible formal power series |
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Canonical name | InvertibleFormalPowerSeries |
Date of creation | 2016-04-27 10:47:14 |
Last modified on | 2016-04-27 10:47:14 |
Owner | pahio (2872) |
Last modified by | pahio (2872) |
Numerical id | 8 |
Author | pahio (2872) |
Entry type | Theorem |
Classification | msc 13H05 |
Classification | msc 13F25 |
Classification | msc 13J05 |
Related topic | RulesOfCalculusForDerivativeOfFormalPowerSeries |