# Counting real roots of bivariate polynomial

For a proof, I'm working with a polynomial $$p(x,m)$$ and I need to show that: For any $$m\in (0,6)$$, $$p_m(x)\equiv p(x,m)$$ has a unique real root in $$(0,1/2)$$. I find that this is true by using Reduce in Mathematica, but I need to understand how to prove it.

Ideally, I'd like to go through the step-by-step process that the Reduce command does, but I haven't been able to understand it (I tried running trace on the Reduce command but the output is impossible for me to understand).

The polynomial $$p$$ is named "auxpoly" in the code below.

P1[x_, m_] :=
2 (-6 + m) m^8 (-27 + 2 m) -
x (-15930 m^7 + 5451 m^8 + 1158 m^9 - 236880 m^6 x + 87728 m^7 x +
46144 m^8 x - 1482048 m^5 x^2 + 616128 m^6 x^2 +
703888 m^7 x^2 - 4460544 m^4 x^3 + 2058112 m^5 x^3 +
6162880 m^6 x^3 - 6414336 m^3 x^4 + 2120192 m^4 x^4 +
34435712 m^5 x^4 - 3538944 m^2 x^5 - 5603328 m^3 x^5 +
124934656 m^4 x^5 - 16908288 m^2 x^6 + 289193984 m^3 x^6 -
12582912 m x^7 + 407371776 m^2 x^7 + 314572800 m x^8 +
100663296 x^9)

P2[x_, m_] := -1836 m^7 + 586 m^8 + 52 m^9 +
4 x (-11808 m^6 + 4200 m^7 + 1528 m^8 - 103752 m^5 x +
40891 m^6 x + 33098 m^7 x - 398592 m^4 x^2 + 181024 m^5 x^2 +
358416 m^6 x^2 - 691200 m^3 x^3 + 303424 m^4 x^3 +
2375360 m^5 x^3 - 442368 m^2 x^4 - 319488 m^3 x^4 +
10032448 m^4 x^4 - 1720320 m^2 x^5 + 26613760 m^3 x^5 -
1572864 m x^6 + 42270720 m^2 x^6 + 36175872 m x^7 + 12582912 x^8)

auxpoly[x_, m_] := (P1[x, m])^2 - 2 x *(m + 2 x)*(P2[x, m]^2)



When I run the Reduce command as follows, I find that for each $$m$$, there is a single real root in the interval $$(0,1/2)$$. I confirm the result by plotting CountRoots for each $$m$$.

Reduce[auxpoly[x, m] == 0 && x > 0 && x <= 1/2 && m > 0 && m < 6, x] • Since you desire a step-by-step solution, why not posting this question on math.stackexchange? Jun 25, 2020 at 6:12
• The code involved uses some heavy machinery such as cylindrical algebraic decomposition (CAD). It will not readily lend itself to human-understandable step-by-step. One can obtain a computer-assisted proof using Discriminant, Roots, and some related functions, but it is not obvious whether that will suit the needs of s-o-s. Jun 25, 2020 at 13:42
• Thanks @DanielLichtblau, that's helpful!
– ilan
Jun 26, 2020 at 5:25

Clear["Global*"]

P1[x_, m_] :=
2 (-6 + m) m^8 (-27 + 2 m) -
x (-15930 m^7 + 5451 m^8 + 1158 m^9 - 236880 m^6 x + 87728 m^7 x +
46144 m^8 x - 1482048 m^5 x^2 + 616128 m^6 x^2 + 703888 m^7 x^2 -
4460544 m^4 x^3 + 2058112 m^5 x^3 + 6162880 m^6 x^3 - 6414336 m^3 x^4 +
2120192 m^4 x^4 + 34435712 m^5 x^4 - 3538944 m^2 x^5 - 5603328 m^3 x^5 +
124934656 m^4 x^5 - 16908288 m^2 x^6 + 289193984 m^3 x^6 -
12582912 m x^7 + 407371776 m^2 x^7 + 314572800 m x^8 + 100663296 x^9)

P2[x_, m_] := -1836 m^7 + 586 m^8 + 52 m^9 +
4 x (-11808 m^6 + 4200 m^7 + 1528 m^8 - 103752 m^5 x + 40891 m^6 x +
33098 m^7 x - 398592 m^4 x^2 + 181024 m^5 x^2 + 358416 m^6 x^2 -
691200 m^3 x^3 + 303424 m^4 x^3 + 2375360 m^5 x^3 - 442368 m^2 x^4 -
319488 m^3 x^4 + 10032448 m^4 x^4 - 1720320 m^2 x^5 + 26613760 m^3 x^5 -
1572864 m x^6 + 42270720 m^2 x^6 + 36175872 m x^7 + 12582912 x^8)

auxpoly[x_, m_] := (P1[x, m])^2 - 2 x*(m + 2 x)*(P2[x, m]^2)


There are four real roots for 0 <= m <= 6

sol = Solve[{auxpoly[x, m] == 0, 0 <= m <= 6}, x, Reals];

Length@sol

(* 4 *)


However, only one of these roots is in the interval 0 <= x <= 1/2

sol2 = Solve[{auxpoly[x, m] == 0, 0 <= m <= 6, 0 <= x <= 1/2}, x, Reals];

Length@sol2

(* 1 *)


The maximum value of x is

max = NMaximize[{Normal[x /. sol2[]], 0 < m < 6}, m]

(* {0.161383, {m -> 0.493441}} *)


Plotting the root as a function of m

Plot[x /. sol2, {m, 0, 6}, Epilog ->
{Red, AbsolutePointSize, Point[{max[[2, 1, -1]], max[]}]},
AxesLabel -> (Style[#, 12, Bold] & /@ {m, x})] • Thanks very much, Bob! Do you know if it's possible to get the step-by-step process that the Solve command performs to get this result? That would be super helpful.
– ilan
Jun 25, 2020 at 4:53
• For very simple problems (not the one that you have) you can use WolframAlpha, e.g., WolframAlpha["solve x^3+x^2+1==0 over reals", {{"ResultOverTheReals", 2}, "Content"}, PodStates -> {"ResultOverTheReals__Step-by-step solution"}]. However, these are not the steps used by Solve`. Jun 25, 2020 at 18:22