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From the way linear/quadratic/cubic convergence of a sequence are
defined, I wonder why they are called linear/quadratic/cubic, in the
sense of some connections to linear/quadratic/cubic functions.Here are the definitions of linear/quadratic/cubic convergence of a
sequence in my words based on WikipediaSuppose that the sequence $\{x_k\}$ converges to the number $L$.
Suppose $q > 1$.When $\lim_{k\to \infty} \frac{|x_{k+1}-L|}{|x_k-L|^q} = μ$ and $μ ∈ (0, 1)$, we say that the sequence (Q-)converges linearly if
$q=1$,
quadratically if $q=2$, and cubically if $q=3$. -
Similarly, how is logarithmic convergence connected to a logarithm
function? The definition of logarithmic convergence is from the same
link to Wikipedia:If the sequences converges sublinearly and additionally $
\lim_{k\to \infty} \frac{|x_{k+2} – x_{k+1}|}{|x_{k+1} – x_k|} = 1, $ then it is said the sequence $\{x_k\}$ converges logarithmically
to $L$.
I found a plot of linear, linear, quadratic and logarithmic rates of convergence for an example in Wikipedia, which seems to
suggest some connection, although it is not clear to me how they are
connected:
Thanks for clarification!
Best Answer
It's simply that $x^q$ is a linear function of $x$ if $q=1$, a quadratic function of $x$ if $q=2$ and a cubic function of $x$ if $q=3$. Convergence "logarithmically" is different, and that definition really doesn't have any necessary connection to a logarithm function.