## 12705 Reputation

8 years, 270 days

## maybe...

Maybe they used Mathematica.

[ just wanted to be malicious :-) ]

Edit. More seriously, using some vector letters (as polygons)

## You may solve eqs wrt all the variables:...

You may solve eqs wrt all the variables:

`eqs := {x - a*y,y - a*x}:solve(eqs);`

` `

## combinat...

n:=3:
Matrix(combinat[permute]( [-1\$n, 1\$n], n) );

## x||i...

seq(x||i,i=1..200);

## N...

You probably mean:

xn:=n^2/(n^2+31*n+228):

x0:=limit(xn,n=infinity);
1
asympt(xn-x0,n,2):
r:=abs(convert(%,polynom)) assuming n>0:
solve(r<eps/2, n, useassumptions) assuming n>0,eps>0:
N:=lhs(%[]);

is( abs(xn-x0)<eps ) assuming eps>0, n>=N;

true

# For integer N, take  N := floor(N)+1

## constant coefficients...

So, you have a homogeneous linear system of ODEs with constant coefficients.
If you know a fundamental system for the solutions:

exp(p*t), exp(q*t), ...

where p,q, ... are distinct complex numbers
then the characteristic polynomial in variable y is

(y - p)(y - q) ...

[if there are multiple roots, the situation is a bit more complicated]

Note that c(t) = 0 cannot appear in a fundamental system
(because of the linear independence).

## indets(eq,anyfunc(identical(t)));...

indets(eq,anyfunc(identical(t)));

## fsolve...

If you must use fsolve, try to restrict the domain.

fsolve({f, g}, {a, c}, a = 10000 .. infinity, c = 0 .. infinity);

Edit.
1. You may use
plots[implicitplot]([f,g], a=-20..20,c=-20..20,color=[red,blue]);
to localize the roots
2. The system seems to be intentionally constructed to have "non-intuitive" solutions.

## forgot...

Yes, obviously for the ics case Maple "forgot" the BesselJ term .

M := diff(T(r), r, r)+(diff(T(r), r))/r+u*T(r)+P*(r^4+r^2) = 0; # u includes the constant

M0 := {M, D(T)(0)=0}:
s:=dsolve(M);

s0:=dsolve(M0);

s0general:=T(r)=BesselJ(0, sqrt(u)*r)*_C2-P*(r^4*u^2+r^2*u^2-16*r^2*u-4*u+64)/u^3;

odetest(s, M);

0
odetest(s0, M0);

{0}
odetest(s0general, M0);

{0}

Edit. If another ics is added e.g.
M01 := {M, D(T)(0)=0, T(0)=a};
then BesselJ appears!

## Groebner...

B:=Groebner[Basis]([x-v, y-v^2], plex(u,v,x,y)):
remove(has,B,[u,v]);

flatt:=proc(L::{list,set})
local a,n; a:=L; n:=-1;
while nops(a)<>n do
n:=nops(a);
a:=map(x->`if`(type(x,{list,set}),op(x),x),a);
od;
op(a)
end:

flatt([1,2,3, {4,5,6}]);

## I or -I...

ex:=-Omega*a*sqrt(2)*sqrt(-Omega^2*a^2-2*k*m+sqrt(Omega^2*a^2*(Omega^2*a^2+4*k*m)))/(-Omega^2*a^2+sqrt(Omega^2*a^2*(Omega^2*a^2+4*k*m))):

-1

So, ex = I or -I
Note that both values may appear, depending on the parameters: e.g. changing a to -a  ==> ex to -ex
ex = I for  e.g. Omega=1, a=2, k=-1, m=1.

Edit. As Markiyan has pointed out, OP says Omega, a, k, m are >0. (I did not see this because the horizontal scroll was absent in my browser). But in this case it is easy to see that ex = -I. In fact, the denominator is obviously >0,  so

must be <0 (because ex^2=-1).

It follows that the argument of ex is - Pi/2 ==> ex = - I.

## Oscillatory...

The solution for your ODEs is very oscillatory.
Maple can easily find a series solution, but it will be almost useless.
I think that the solution itself would be useless if found.

Please look at the following similar simple example:

Digits:=30: Order:=10:
a:=10^6:
s1:=dsolve({diff(y(t),t)=cos(a*t)*y(t), y(0)=1}, y(t), series):
s2:=dsolve({diff(y(t),t)=cos(a*t)*y(t), y(0)=1}, y(t)):
ss1:=convert(rhs(s1),polynom);

ss2:=rhs(s2); #exact

plot(ss2, t=0..1);  #exact

plot( ss1, t=0..1); #series

## - You must give numerical values to para...

- You must give numerical values to parameters, e.g.

omega:=1; M:=1; N:=1; N1:=1; Delta:=1;

(you probably did).

- Choose first a smaller interval, e.g.

plots[odeplot](res,[[t,(Re(w(t)))]],0..5,axes=boxed,titlefont=[SYMBOL,14],font=[1,1,18],color=black,linestyle=1,tickmarks=[3, 4],font=[1,1,14],thickness=2,titlefont=[SYMBOL,12]);

(for larger intervals it will take several minutes)

It works.

## unapply and proc...

I think that unapply does not work properly if its first argument is a procedure.

So,

g := unapply(x -> theta*x, theta);

also does not work.
unapply is not builtin, so it can be examined.

A curious fact is that f and g are apparently the same, according to print or lprint,
but they have distinct internal structures according to
dismantle(eval(f)); dismantle(eval(g));

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