## dharr

Dr. David Harrington

## 6426 Reputation

20 years, 25 days
University of Victoria
Professor or university staff

## Social Networks and Content at Maplesoft.com

I am a retired professor of chemistry at the University of Victoria, BC, Canada. My research areas are electrochemistry and surface science. I have been a user of Maple since about 1990.

## more efficient version...

@dharr This version is more efficient to generate the whole sequence up to some value, and uses the q-series expansions directly.

```congruentlist := proc(nmax::posint)
local mgmax, ngmax, m2max, m4max, nn, g, theta2, theta4, gth2, gth4,
an, bn, n, m, i, j, q, c, x;
description "Find list of congruent numbers up to possible value nmax using Tunnell's theorem";
# ensure enough terms in the q-series for nmax to be reliable
ngmax := ceil(sqrt(nmax/8));
mgmax := ceil((sqrt(nmax) + 1)/4);
m2max := ceil(sqrt(nmax/2));
m4max := ceil(sqrt(nmax/8));
g := add(add((-1)^n*q^((4*m + 1)^2 + 8*n^2), m = -mgmax .. mgmax), n = -ngmax .. ngmax);
theta2 := 1 + 2*add(q^(2*m^2), m = 1 .. m2max);
theta4 := 1 + 2*add(q^(4*m^2), m = 1 .. m4max);
gth2 := expand(g*theta2);
an := [seq(coeff(gth2, q, i), i = 1 .. nmax, 2)];
gth4 := expand(g*theta4);
bn := [seq(coeff(gth4, q, i), i = 1 .. nmax/2)];
j := 0;
for n to nmax do
# divide out largest square and use squarefree residue
nn:= mul(map(x -> x[1]^(x[2] mod 2), ifactors(n)[2]));
if nn::odd and (an[(nn+1)/2] = 0) or nn::even and (bn[nn/2] = 0) then
j := j + 1;
c[j] := n;
end if;
end do;
convert(c, list)
end proc:```

## philosophy...

The method given was tailored for that specific example. It gives a zone diagram for when f1 is greater/less/equal than f3 for the two key combined parameters of the problem, which are plotted on the two axes. At the heart of the problem is the non-dimensionalized function Lambda(Gamma), which is "dressed up" with the additional parameters of the problem. So the manipulations were to find the relevant key parameters. You say that the scaling was just to plot it, but that amounts to the same thing - finding a single plot that encapsulates the data so you don't need a series of plots for different parameters. The key is to solve for when f1 = f3 (still true for scaled versions), which gives a line separating the different zones - then one can test the various regions to see whether they are for f1>f3 or f1<f3. If I undestand what you want, its give a complete solution for what combinations of parameters lead to f1 </>/= f3.

The same philosophy applies to partial derivatives, because these are all dressed up versions of diff(Lambda(Gamma),Gamma) via the chain rule. For combinations of functions, you might have more key parameters and a more complicated situations, or you might have just two.

You ask how to fix complicated_comparison.mw, but really it is giving you what you asked for. The LambertW appears for equations involving the form x = exp(-a*x), which is a key part of your approximate version, with x = f3. There isn't a simpler analytical solution. I could have solved for f3 and made a plot of LambertW functions, but it is simpler to use the parametric plot. (My experience is that LambertW functions are tricky because of the different branches.) But the key point is that this is the line on some plot. Notice that in asking for solve to use the various inequalities you are asking for all cases of these parameters, and not products/ratios of them. If you first figure out the key parameter combinations and call them, say a and b, then this method might be more useful.

complicated_comparison2.mw didn't work because there are still parameters in g__1sc; a more careful analysis/manipulation of the parameters is required.

(I didn't look closely at partial_derivatives.mw)

## calculating the sides...

@dharr Probably there is a more efficient way but this works.

Calculate the sides of  triangle having the congruent number n.
Procedure congruent is in the startup code.

 >
 > sides:=proc(n::posint)   local a,b,c,i;   if not congruent(n) then error "%1 is not a congruent number", n end if;   for i do     a:=NumberTheory:-CalkinWilfSequence(i);     b:=2*n/a;     c:=sqrt(a^2+b^2);   until c::rational;   a,b,c end proc:
 >

 >

 >

## sides from area...

@JAMET congruent(3) returns false, which is correct; neither Wikipedia nor OEIS list 3 as congruent. For the sides calculation see below.

## Introduced in 2023...

@jud I am using version 2023, which has it, but 2022 doesn't.

## my version...

In response to your email request, here is my version.

Test if n is a congruent number by Tunnell's theorem.
(Assumes the Birch and Swinnerton-Dyer conjecture is true.)
Procedure congruent is in startup code.

 >

We calculate the number of integer solutions An, Bn, Cn, Dn for the following equations.
For a squarefree integer:
If n is odd then it is congruent if
If n is even then it is congruent if
If n is not square-free, divide out largest square first.

 >

From Wikipedia, the first congruent numbers are

 >
 >
 >

 >

The code in the worksheet should work in older versions of Maple. In 2023 it can be done a little shorter as:

```congruent := proc(n::posint)
local nn, An, Bn, c1, c2, c3, c4, x, y, p, q, mx, my, mz;
description "Determine if n is a congruent number using Tunnell's theorem";
# divide out largest square and use squarefree residue
nn := mul(map(x -> x[1]^(x[2] mod 2), ifactors(n)[2]));
if nn::odd then
c1, c2, c3, c4 := 2, 1, 32, 8
else
c1, c2, c3, c4 := 8, 2, 64, 16
end if;
An := 0;
Bn := 0;
for x from 0 while (p := nn - c1*x^2) >= 0 do
for y from 0 while (q := p - c2*y^2) >= 0 do
if issqr(q/c3) then
# count all solutions e.g., for x nonzero there two possibilities of opposite sign
mx := signum(x) + 1;
my := signum(y) + 1;
mz := signum(q) + 1;
An := An + mx*my*mz;
end if;
if issqr(q/c4) then
mx := signum(x) + 1;
my := signum(y) + 1;
mz := signum(q) + 1;
Bn := Bn + mx*my*mz;
end if;
end do
end do:
evalb(2*An = Bn);
end proc;

```

1- How can solve these two ODE(s) and  plot phi vs x in one phi-x coordinate?

Just solve them separately and generate two plots, in variables p1 and p2. Then display(p1,p2) will put them on the same plot.

2- How can I plot phi-x in a symmetric interval of x (for example, from x=-100..100).

The solver can go forwards and backwards from the initial point. But you didn't explain the significance of x=0 so it is not clear how the x axis is defined.

3- How can I export data of plot of these ODEs in the form of two distinct ascii table? (I couldn’t export data of the last figure in your last uploaded worksheet in the form of a ascii table)

I don't know why the export didn't work. If you upload a worksheet containing the error I can take a look.

## exporting plot data...

@mary120 if you assign the odeplot to a variable p, then M:=plottools:-getdata(p)[3]; gives a Matrix with the data. The matrix can then be exported with ExportMatrix to varrous  file types, e.g., target = csv will be comma separated.

## phi vs x plot...

@mary120 To plot phi vs x just use

`odeplot(ans1,[x(phi),phi],1e-2..0.5);`

This goes through the origin because of the initial condition x(0)=0. But your plot doesn't go through the origin so I don't understand what you really want. Note the slope is wrong, so you probably need diff(x(phi),phi)=-integrand

Integral-New.mw

Here is something, but it is unclear how the x axis is constructed - what does x=0 mean on your plot?

something.mw

@Carl Love My original example was a graph with some randomly chosen edges. Later @Anthrazit wanted it to be a complete graph, and so I gave a version that does the complete graph for the same points, which does give the same path and distance as you find, see the worksheet TravelingSalesmanCompleteGraph.mw above.

I was also assuming that the start and end vertices were prescribed, and were to be given as the first and last in the list (since @Anthrazit mentioned something about nearest and outermost bolts), rather than just the shortest of all Hamiltonian paths.

## missing multiplication...

@Saha   You have gamma(Theta - Theta[a]) instead of gamma*(Theta - Theta[a])

## syntax...

@sand15 Yes! That syntax error can be avoided by diff(..., [eta\$(m-2)]) , which returns the undifferentiated function. But probably not what the OP intended.

## syntax...

If I comment out with # the line setting values to various variables, then look at the output of the main loop I notice:

rps1.mw

1. Theta' is being interpreted as a derivative with respect to x. Probably you want to use declare(prime=eta)

2. You have multiplication before square brackets [] in two places, which Maple does not know how to interpret - if these are just for grouping you should use ().

3. The derivative with respect to eta m-2 times has not worked - the "d" should appear as upright, not italics. Use the calculus palette to form this, or use diff( ... , eta \$ (m-2))

4. You have both Theta and Theta[a] and Theta with other subscripts. If it has subscripts you should not use Theta without subscripts as well.

5. If you are intending Theta without subscripts to be a function of eta you need to write Theta(eta)  or use declare so that Maple does not think it is a constant when it is differentiating.

Perhaps after fixing these, it will be clearer what you want to do, but at the moment I do not understand.

## click on plot?...

In Windows, if you click on the plot, the controls appear.

## CompleteGraph version...

@Anthrazit Here's the CompleteGraph version (usually in traveling salesman problems there are not roads between every combination of cities). If you want shortest paths between two vertices that don't have to visit all vertices, DijkstrasAlgorithm or BellmanFordAlgorithm can be used - both can find shortest paths from vertex 1 to all other vertices, and then you can use an undirected graph - a path will not revisit an edge). If that is what you want I can set it up.

TravelingSalesmanCompleteGraph.mw

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