## 12605 Reputation

8 years, 249 days

## Just for fun...

@Thomas Richard Sometimes the situation is reversed!

 > f:=polylog(2, -1 - s) + polylog(2, (1 + s)/(s + 2));
 (1)
 > simplify(f);  # nothing
 (2)
 > simplify(convert(f, dilog));  # That's it!
 (3)

## parametric...

@sursumCorda So, you want an explicit solution instead of a parametric one (t3 being the parameter).
This could be very difficult (or even impossible) to obtain in general.

## It would be nice to tell us how the equa...

It would be nice to tell us how the equation was obtained.

Here is a (simpler) parametric one (Klimek G.&M. - Discovering curves and surfaces with Maple, Springer 1997.)

```plot3d([(4+3.8*cos(y))*cos(x),
(4+3.8*cos(y))*sin(x),
(cos(y)+sin(y)-1)*(1+sin(y))*log(1-Pi/10*y)+7.5*sin(y)],
x=-Pi .. Pi, y=-Pi .. Pi, color=[sin(x+1)+cos(y), sin(y), 0.3],
light=[85, 50,  0.6, 0.8, 0.2], orientation=[47, 55],
style=patchnogrid, axes=none);```

## ?...

Your function of Pe also depends on several variables and some arbitrary functions!
How can you imagine that the limit could be computed?

## assume or assuming...

@mmcdara For me, it's not about assume or assuming.
For the same assumptions, assumptions := a>0, b>0, a*b < 1:
assuming
gives the same results.
Also, in Maple 2022, assumptions := a>0, b>0, b < 1/a:
==> all answers are true (But it seems that in 2023, one answer is FAIL !!).

So, everything seems to be about weakness!

## Unfortunately...

@Carl Love  Unfortunately the assume facility is very weak here.

```restart;
# _EnvTry := true;  # useless
assume(a>0, b>0, a*b<1);
is( (1 + sqrt(1-a^2*b^2))/(a*b) >0 ); # FAIL   ?
is( (1 + sqrt(1-a*b))/(a*b) >0 );     # FAIL   ?
is( (1 + sqrt(1-a^2*b^2)) >0 );       # false ??
is( (1 - sqrt(1-a^2*b^2)) >0 );       # false ??
is( (1 - sqrt(1-a*b)) >0 );           # true
is( (1 + sqrt(1-a*b)) >0 );           # true
```

## c1, c2...

@nm The general solution of the ode  y'' + y = 0  is y = c1*sin(x) + c2*cos(x)  but also
y = c1*(2*sin-7*cos(x)) - c2*(sin(x)-3*cos(x)).

## Remarks...

@dharr a) A square complex matrix C has not a square root iff its Jordan form has a Jordan block having dimension > 1 and eigenvalue 0.

b) For C as in the file (40x40), M := evalf[n](MatrixPower(C, 1/2)), eps = Norm(M^2-C)  then:
n=14-->eps=0.003;  n=16-->eps=2e-4;   n=18-->eps=1e-6;  n=25--> eps=2e-13;  n=30-->eps= 2e-15.

## odetest...

Use odetest to check the solution. Yes, it's a bug.

```restart;
ode:=[diff(x(t), t, t) + 2*diff(x(t), t)/sqrt(L*C) + x(t)/(L*C)];
s5 := dsolve(ode, [x(t)]);
odetest(s5, ode);
s6 := dsolve(ode, [x(t)], method = laplace);
odetest(s6, ode);   # bug
```

## Not the same thing...

@Christian Wolinski The problem is that {a>0, b>0, c>0, d>0} is far for being equivalent to {e1>0, e2>0, e3>0, e4>0}.

## G & E...

@tomleslie
G is the centroid of the (inscribed) quadrilateral P1P2P3P4.
E is the Euler center of  the quadrilateral P1P2P3P4  defined as the point in the intersection of the four Euler circles of the triangles P1P2P3, ..., P4P1P2. (E exists!).

Note that OG = (OP1+OP2+OP3+OP4)/4, OE = (OP1+OP2+OP3+OP4)/2,  OPk  being vectors.

## Proof...

@Alex0099  1. The order of summation does not matter, the summand being symmetric.

2. The proof could be very tricky. Probably here the Residue Theorem should be useful.

## restart...

@Thomas Dean  In Windows, after a restart everything is OK. Sorry, I don't use Linux.

## type...

@emendes  procedure is a type, so, its use is covered.

## Draghilev...

AFAIK the Draghilev method is to find a natural parametric representation of a curve given as an intersection of n surfaces in (n+1) unknowns. That is what Rouben did, but using direct computations.

(I was never able to find the original article of Draghilev, but there are many places -- even in this site -- where the method is used).

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