☆ Π MTH ~CLVii~✧⁰¹⁵⁷ Π∞ßτ З³²² ⁰⁰כ
Studying the Koch curve can be unsettling. The curve starts with 2 points
•----•
After applying the algorithm, the 2 points become 5 points
•
/ \
▪︎--▪︎ ▪︎--▪︎
When we apply the algorithm again, the 5 points become 17 points, because each segment generates 3 points
\[4×3+5=17\]
And the 17 points generate 16 segments, where for each one we have 3 points, so that
\[16×3+17=65\]
Finally, we see the recursive law
\[p=(u-1)×3+u\]
Which results in
\[p=4u-3\]
Generating the OEIS A052539
\[2\space5\space17\space65\space257 \dotsc\]
However, the recursive formulation
\[a_{n+1}=4a_{n}-3;n\ge0\]
is unnecessary because the direct formulation is evident
\[a_n=4^n+1;n\ge0\]
Note that this expression can be deduced using first-order linear recurrence; so that the sequence
\[a_0\space a_1\space a_2\space\dotsc \space a_n\space \dotsc \]
Where the terms are obtained by the recurrence relation
\[a_n=a_{n-1}×k+r; n\ge1;k\neq1\]
So it is a fact that
\[\begin{align}
a_0&=a_0\\
a_1&=a_0×k+r\\
a_2&=a_1×k+r\\
&=(a_0×k+r)×k+r\\
&=a_0×k^2+rk+r\\
&=a_0k^2+r(k+1)\\
&\vdots\\
a_n&=a_0k^n + r(k^{n-1}+k^{n-2}+\dotsc+1)
\end{align}\]
Using the formula for a finite geometric sequence
\[S_n=\alpha_1\frac{q^n-1}{q-1}\]
To sum the terms
\[\overbrace{1\space k\space k²\space\dotsc\space k^{n-1}}^n\]
we have
\[a_n=a_0k^n+r\frac{k^n-1}{k-1}\]
Thus, for the sequence
\[2\space5\space17\space65\space257 \dotsc\]
We have
\[\left\{\begin{align}
a_0 &=2\\
k&=4\\
r&=-3
\end{align}\right.\]
Hence
\[a_n=2×4^n+(-3)\frac{4^n-1}{4-1}\]
It follows that
\[a_n=4^n+1;n\ge0\]
\[■\]
αΩ
~ ∇ ~
#math #art
[ BRAZIL ⋅ З³²² LND Lunedì χViii Maggio MMχχVi ⁰⁰כ ]