On Thu, Nov 17, 2011 at 5:24 AM, Bill Gosper <billgosper@gmail.com> wrote:
On Mon, Nov 14, 2011 at 3:22 PM, Bill Gosper <billgosper@gmail.com> wrote:
http://gosper.org/islands.png shows four triadic dragons sharing an endpoint, and fanned out at angular intervals π/6, π/3, and π/2. If we suppose each has unit area, it is a minor puzzle to compute the distance between endpoints (12^(1/4)), and the (slightly greater) diameter, which I haven't bothered to rederive. Perhaps the easiest area derivation is to note that the dragons tile the (infinite) plane in a triangular grid, with 3/2 dragons/triangle. This is a case of using infinity to answer a finite question, and is unrelated to the dragon's infinite perimeter, e.g.
Small constant #1: What is the diameter minus endpoint span.
This is a fairly easy exercise.
Notice the purple Hawaiianoid chain of apparently similar islands formed by the purple dragon minus the blue one, and the (necessarily) congruent white chain between the green and gold dragons. While trying to characterize these Islands, Corey & Julian noticed something peculiar.
Define A(ø) := the area of intersection of two coterminous unit area tdragons fanned at angle ø. A(π/3)=0 by virtue of the plane tiling. A(π/6) appears to be some simple function of the area of an island chain. But maybe not an island! On very close inspection, A(π/2) is very slightly positive. This is not a positioning artifact--in the fractal limit, coterminous dragons infinitely entwine, and cannot be slid apart without rotation. This is not visually obvious because they are based on a log spiral with a scale gain of 729 per turn. (kth order dragon ~ (rt3 e^(i pi/6))^k. k=12 makes one turn.)
It thus appears that the islands are very slightly dissimilar!
No, they are all similar, and joined by isthmi similar to http://gosper.org/isthmus.png , which appears to be infinitely nested, if not downright fractal. This image was enabled by the exact "rational" triadic dragonskin function
Clear[Dskin]; Dskin[t_, a1_: 1, a0_: 0] := Dskin[t, b1_: 1, b0_: 0] = (Dskin[t, s1_: 1, s0_: 0] = ((a0 - s0)/(s1 - a1)); Module[{t2 = 2*t, n}, n = Floor[t2]; t2 -= n; ComplexExpand[Switch[n, 0, (I/Sqrt[3] + 1)/2*Dskin[t2, a1*(I/Sqrt[3] + 1)/2, a0], 1, 1 + (I/Sqrt[3] - 1)/2* Dskin[1 - t2, (I/Sqrt[3] - 1)/2*a1, a1 + a0], 2, 1]]])
completely analogous to the exact dragon function, since the skin of a triadic dragon is just a dyadic dragon dimension-reduced by unfolding to 2 π/3 instead of π/2. E.g., In[147]:= Dskin[7/22]
Out[147]= 157/542 + (265 I)/(542 Sqrt[3]) --rwg
Small constant #2: What is A(π/2)?
Not easy!
Small constant #3: What is the least positive ϵ for which A(π/2+ϵ)=0?
Early experiments look like roughly π/200.
--rwg
I can't experimentally distinguish this quantity from π/144. http://gosper.org/isthmus22.png is a contact or near contact with a step size of 2^-22, Cmd was Timing[ListLinePlot[{Table[ Dskin[t], {t, 3/4 - (8 + 5/8)/1024, 3/4 - (8 + 3/8)/1024, 1/64/16/16/8/8/4}], Table[Dskin[t], {t, Floor[128^2/Sqrt[3]*(14 + 3/4)/16]/128^2, Ceiling[128^2/Sqrt[3]*(14 + 37/49)/16]/128^2, 1/64/16/16/8/8/4}]/Exp[I*\[Pi] (1/6 + 1/144)]} /. z_?NumericQ -> {Re@z, Im@z}, AspectRatio -> Automatic, Axes -> None]] which took ~45 min with a(n over?)populated hash table. --rwg Good thing I checked: I pasted the old isthmus url and added the "22" http://gosper.org/isthmus22.png <http://gosper.org/isthmus.png> but this still points to the old url!