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torus:= (x^2+y^2+z^2 + R^2-r^2)^2 - 4*R^2*(x^2+y^2):




plots:-implicitplot3d(torus1=0, x=-6..6,y=-6..6,z=-6..6, numpoints=5000, scaling=constrained, view=-2..2);


sol1:=solve(torus1<0, [x,y,z]); # It should be easy for Maple

[[x = 0, y < -3, -5 < y, z < (-y^2-8*y-15)^(1/2), -(-y^2-8*y-15)^(1/2) < z], [x = 0, y < 5, 3 < y, z < (-y^2+8*y-15)^(1/2), -(-y^2+8*y-15)^(1/2) < z]]


eval(torus1, [x=3,y=2,z=0]); # in the interior



eval(sol1, [x=3,y=2,z=0]);   # ???

[[3 = 0, 2 < -3, -5 < 2, 0 < (-35)^(1/2), -(-35)^(1/2) < 0], [3 = 0, 2 < 5, 3 < 2, 0 < (-3)^(1/2), -(-3)^(1/2) < 0]]




Download Bug-solve_ineqs.mw



plots:-inequal([x^2+y^2<100, x+y>Pi]);      # ?,  evalf(Pi) ok

Error, (in ReasonableDomain:-Implicit) invalid input: ReasonableDomain:-Recorder:-AddPoint expects its 2nd argument, point, to be of type list(numeric), but received [Pi, 0]


plots:-inequal([x^2+y^2<100, x+y>sqrt(3)]); # ?

Error, (in ReasonableDomain:-Implicit) invalid input: ReasonableDomain:-Recorder:-AddPoint expects its 2nd argument, point, to be of type list(numeric), but received [3^(1/2), 0]


solve({x^2+y^2<100, x+y>Pi});               # ?

Warning, solutions may have been lost


solve({x^2+y^2<100, x+y>sqrt(10)});         # ?

Warning, solutions may have been lost


solve({x^2+y^2<100, x+y>4}, [x,y]);         # OK

[[x < 2+46^(1/2), 2-46^(1/2) < x, y < (-x^2+100)^(1/2), 4-x < y], [x = 2+46^(1/2), y < -2+46^(1/2), 2-46^(1/2) < y], [x < 10, 2+46^(1/2) < x, y < (-x^2+100)^(1/2), -(-x^2+100)^(1/2) < y]]


solve({x^2+y^2<100, x+y>a}, [x,y]) assuming 3<a, a<5;              #?



solve({x^2+y^2<100, x+y>a}, [x,y], parametric) assuming 3<a, a<5;  # OK

[[x = (1/2)*a+(1/2)*(-a^2+200)^(1/2), (1/2)*a-(1/2)*(-a^2+200)^(1/2) < y, y < -(1/2)*a+(1/2)*(-a^2+200)^(1/2)], [(1/2)*a-(1/2)*(-a^2+200)^(1/2) < x, x < (1/2)*a+(1/2)*(-a^2+200)^(1/2), a-x < y, y < (-x^2+100)^(1/2)], [(1/2)*a+(1/2)*(-a^2+200)^(1/2) < x, x < 10, -(-x^2+100)^(1/2) < y, y < (-x^2+100)^(1/2)]]


Download bugs-irrationals.mw

About eliminate(...)


This post is motivated by a recent answer where I needed a necessary and sufficient condition for three straight lines in space be concurrent. I had to use determinants because the eliminate command did not provide the correct answer.
Investigating the cause, I saw that eliminate uses an heuristic algorithm, instead of using Groebner bases (when possible).

Here is an example.

We want to eliminate the unknowns x an y in the system

a*x + y = 0,  b*x+y+1 = 0, c*x+2*y = 0


sys:=[a*x + y, b*x + y + 1, c*x + 2*y];

[a*x+y, b*x+y+1, c*x+2*y]


eliminate(sys, [x,y]);

[{x = 0, y = 0}, {1}]


So, apparently, the elimination is not possible, i.e. for each triple (a,b,c), the system in x and y is incompatible.
This is not true. For example,



[x+y, 3*x+y+1, 2*x+2*y]


eval(%, [x=-1/2, y=1/2]);

[0, 0, 0]


eliminate  obtained its result this way (just like a superficial human):

solve(sys[[1,3]], [x,y]);
eval(sys[2],%[]); # The result obtained by eliminate

[[x = 0, y = 0]]




Now, the correct result (also by hand):

solve(sys[[1,2]], {x,y});

{x = 1/(a-b), y = -a/(a-b)}






So, for c = 2*a  (and a <> b)  the system in x,y  is compatible.


This result can be obtained with Groebner bases.

Groebner:-Basis(sys, plex(x,y,a,b,c));

[2*a-c, 2*b*y-c*y-c, c*x+2*y, b*x+y+1]


remove(has, %, {x,y});



Note that it is more efficient to use lexdeg([x,y], [a,b,c])  instead of plex(x,y,a,b,c).

Groebner:-Basis(sys, lexdeg([x,y], [a,b,c]));

[2*a-c, 2*b*y-c*y-c, c*x+2*y, b*x+y+1]


The conclusion is that eliminate should use internally Groebner:-Basis for polynomial systems.
Until then, we can use it ourselves!


Maple strings contain extended ASCII characters (codes 1..255). 
The international characters such as  î, ș, Å, Ø ,Ă, Æ, Ç are multi-byte encoded. They are ignored by the Maple engine but are known to the user interface which uses the UTF-8 encoding.
The package StringTools does not support this encoding. However it is not difficult to manage these characters using Python commands (included in the Maple distribution now).
Here are the UTF-8 versions of the Maple procedures length and substring.
You may want to improve these procedures, or to add new ones (my knowledge of Python is very limited).

LEN:=proc(s::string) Python:-EvalFunction("len",s) end proc:

SS:=proc(s::string, mn::{integer, range(({integer,identical()}))})
  local m,n;
  if type(mn,integer) then m,n := mn$2 else m:=lhs(mn); n:=rhs(mn) fi; 
  if m=NULL then m:=1 fi; if n=NULL then n:=-1 fi;
  if n=-1 then n:=LEN(s) elif n<0 then n:=n+1 fi;
  if m=-1 then m:=LEN(s) elif m<0 then m:=m+1 fi;
  Python:-EvalString(cat("\"",  s, "\"", "[", m-1, ":", n, "]"  ));
end proc:

LEN("Canada"), length("Canada");
                              6, 6

LEN(s), length(s);
                              7, 8

SS(s, 1..), SS(s, 1..-3), SS(s, 1..1), SS(s, -7..2), SS(s,1), SS(s,-1); 
            "România", "Român", "R", "Ro", "R", "a"

diff(abs(z), z)  returns abs(1, z)  and the latter, for a numeric z, is defined only for a nonzero real z.
However,  functions such as abs(I+sin(t)) are (real) differentiable for a real t and diff should work. It usually does, but not always.

f:= t -> abs(GAMMA(2*t+I)):  # We want D(f)(1)
evalf(%);  # Error, (in simpl/abs) abs is not differentiable at non-real arguments
D(f)(1); simplify(%); 
evalf(%);   # 0.3808979508 + 1.161104935*I,  wrong

The same wrong results are obtained with diff instead of D

diff(f(t),t):   # or  diff(f(t),t) assuming t::real; 
simplify(%); evalf(%);   # wrong, should be real

To obtain the correct result, we could use the definition of the derivative:

limit((f(t)-f(1))/(t-1), t=1); evalf(%); # OK 
fdiff(f(t), t=1);    # numeric, OK



Note that abs(1, GAMMA(2 + I)); returns 1; this is also wrong, it should produce an error because  GAMMA(2+I) is not real;

Let's redefine now `diff/abs`  and redo the computations.

`diff/abs` := proc(u, z)   # implements d/dx |f(x+i*y|) when f is analytic and f(...)<>0
local u1:=diff(u,z);
1/abs(u)*( Re(u)*Re(u1) + Im(u)*Im(u1) )
f:= t -> abs(GAMMA(2*t+I));
D(f)(1); evalf(%);   # OK now



Now diff works too.

simplify(%); evalf(%);   # it works

This is a probably a very old bug which may make diff(u,x)  fail for expressions having subespressions abs(...) depending on x.

However it works  using assuming x::real, but only if evalc simplifies u.
The problem is actually more serious because diff/ for Re, Im, conjugate should be revized too. Plus some other related things. 

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