Is it possible to represent $$ \sqrt[3] {7\sqrt[3]{20}-1} =\sqrt[3]{A}+\sqrt[3]{B}+\sqrt[3]{C}$$ with rational $A,\,B,$ and $C?$
- $\begingroup$ Are you basing your question on $$\sqrt[6]{7\sqrt[3]{20} - 19} = \sqrt[3]{\frac{5}{3}} - \sqrt[3]{\frac{2}{3}}$$ and the similar identities in this post? $\endgroup$Tito Piezas III– Tito Piezas III2015-04-14 03:07:31 +00:00Commented Apr 14, 2015 at 3:07
- $\begingroup$ @Tito Piezas III: Thank you for the reference. I have not known those. I don't immediately see how to answer my question, using the methods from the references in this post. $\endgroup$user64494– user644942015-04-14 04:09:38 +00:00Commented Apr 14, 2015 at 4:09
- $\begingroup$ After finding my answer, I think there is a connection between, $$\sqrt[3] {-1+7\sqrt[3]{20}} =\sqrt[3]{\frac{16}{9}}+\sqrt[3]{\frac{100}{9}}-\sqrt[3]{\frac{5}{9}}\tag1$$ and $$\sqrt[3] {-19+7\sqrt[3]{20}} =\sqrt[3]{\frac{4}{9}}+\sqrt[3]{\frac{25}{9}}-\sqrt[3]{\frac{80}{9}}\tag2$$ Where did you find the LHS of $(1)$? $\endgroup$Tito Piezas III– Tito Piezas III2015-04-14 05:45:54 +00:00Commented Apr 14, 2015 at 5:45
- $\begingroup$ @Tito Piezas III: Share you opinion. The question originates from math folklore. $\endgroup$user64494– user644942015-04-14 06:49:22 +00:00Commented Apr 14, 2015 at 6:49
1 Answer
Yes.
$$ \sqrt[3] {-1+7\sqrt[3]{20}} =\sqrt[3]{\frac{16}{9}}+\sqrt[3]{\frac{100}{9}}-\sqrt[3]{\frac{5}{9}}\tag0$$
Solution: More generally, given the three roots $x_i$ of any cubic equation,
$$x^3+ax^2+bx+c=0\tag1$$
then sums involving the cube roots of the $x_i$ can be given in the simple form,
$$(u+x_1)^{1/3}+(u+x_2)^{1/3}+(u+x_3)^{1/3} = \big(w+3\,\sqrt[\color{blue}6]{d}\big)^{1/3}$$
where $u,w$ are the constants,
$$u = \frac{ab-9c+\sqrt{d}}{2(a^2-3b)}\tag2$$
$$w = -\frac{(2a^3-9ab+27c)+9\sqrt{d}}{2(a^2-3b)}\tag3$$
and $d$ is,
$$d = \tfrac{1}{27}\Bigl(4(a^2-3b)^3-(2a^3-9ab+27c)^2\Bigr)\tag4$$
Example: For your question, we have,
$$w=-1, \quad d =\frac{7^6\cdot20^2}{3^6}$$
and subbing these into $(3),(4)$, and using $Mathematica$ to simplify, we get $b,c$ for arbitrary $a$ as,
$$b= \tfrac{1}{9}(-343+3a^2)$$
$$c= \tfrac{1}{27}(-2058-343a+a^3)$$
Substituting $b,c,d$ into $(2)$ and $(1)$, we find that,
$$u=\tfrac{1}{9}(37+3a)$$
and $(1)$ factors as,
$$ (7 + a + 3 x) (14 + a + 3 x) (-21 + a + 3 x) = 0\tag5$$
giving $x_1, x_2, x_3$. The expression $u+x_1$ simplifies as just $\frac{16}{9}$ and similarly for $x_2, x_3$, thus resolving into the numerical relation $(0)$ given above.
- $\begingroup$ thats a nice identity involving cube roots of roots of a cubic equation. +1 $\endgroup$2015-04-14 16:25:06 +00:00Commented Apr 14, 2015 at 16:25
- $\begingroup$ @ParamanandSingh: Going to fifth powers, I have tried and have not found any example of $\sum_{n=1}^5 (u+x_n)^{1/5} = (w+\sqrt[k]{d})^{1/5}$ where the $x_i$ are five irrational roots of a quintic. $\endgroup$Tito Piezas III– Tito Piezas III2015-04-15 02:33:43 +00:00Commented Apr 15, 2015 at 2:33
- $\begingroup$ How do you get $a$? $\endgroup$Frank– Frank2016-05-31 15:30:46 +00:00Commented May 31, 2016 at 15:30
- $\begingroup$ @Frank For OP's particular problem, then $a$ is arbitrary. $\endgroup$Tito Piezas III– Tito Piezas III2016-05-31 15:56:16 +00:00Commented May 31, 2016 at 15:56
- $\begingroup$ Sorry, but I am still not understanding what you mean by arbitrary $a$. Do you actually find the value of $a$? Or is it just a random number? (Both are definitions of arbitrary) $\endgroup$Frank– Frank2016-05-31 16:15:54 +00:00Commented May 31, 2016 at 16:15