Why is \( T_k \) the Unique Polynomial of Degree \( k \) with These Properties?

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SUMMARY

The discussion centers on the uniqueness of the \( k \)-th Taylor polynomial \( T_k \) for a \( k \)-times continuously differentiable function \( f: U \rightarrow \mathbb{R} \) defined on an open set \( U \subset \mathbb{R}^n \). Participants demonstrate that \( T_k \) is the only polynomial of degree \( k \) satisfying the conditions \( T_k(0) = f(x_0) \) and the equality of partial derivatives at zero. The conversation emphasizes the application of the product rule in differentiation and the importance of correctly indexing summation variables in the polynomial's formulation.

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  • #31
Klaas van Aarsen said:
Anyway, consider $\frac{\partial}{\partial{x_{i_1}}}\ldots \frac{\partial}{\partial{x_{i_m}}}T_k(0)$.
So we differentiate $m$ times.
It means that all terms with an order lower than $m$ vanish due to differentiation.
And all terms with an order higher than $m$ that did not already vanish due to differentiation, will vanish when we substitute $0$.
That leaves the terms of order $m$, and only those that are a match with $x_{i_1}\cdot\ldots\cdot x_{i_m}$ will remain.
That is, all permutations of it.
And there are $m!$ permutations... 🤔
Ahh I see! And so the second property for $T_k$ follows!

For the uniqueness, we have to show that the coefficients of $T_k$ are uniquely defined, right? :unsure:
 
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  • #32
mathmari said:
Ahh I see! And so the second property for $T_k$ follows!

For the uniqueness, we have to show that the coefficients of $T_k$ are uniquely defined, right?

Yep. (Nod)
 
  • #33
Klaas van Aarsen said:
Yep. (Nod)

Great! Thank you! 🤩
 

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