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I was trying to see if I could apply the method below to solve a certain subclass of the class of cubic equations with only one real root, and I came up with a block whose cause I couldn't find.

OK, so let $$y=ax^3+bx^2+cx+d,$$ where $a,b,c,d$ are real and $a

e 0.$ Then $y'=3ax^2+2bx+c.$ Now the plan is to consider those cubics as defined above for which $y'$ has the same sign for all $x.$ This would ensure that an inverse function $$x=f(y)$$ exists, and the plan is to get this function by integration. Let's continue.

Now, for $y'>0,$ without loss of generality, we must have $(2b)^2-4(3a)c\lt 0,$ or $$3ac-b^2>0.$$ Thus, with $k^2=3ac-b^2,$ we have $$y'=3a\left[\left(x+\frac{b}{3a}\right)^2+\frac{k^2}{9a^2}\right],$$ by attempting to complete squares. It then follows, since $y'

e 0$ for all $x,$ that $$x=\int{\frac{\mathrm d y}{3a\left[\left(y+\frac{b}{3a}\right)^2+\frac{k^2}{9a^2}\right]}}$$ exists for all $y.$ This integral can be easily evaluated to give $$\frac1k \arctan{\left(\frac{3ay}{k}+\frac{b}{k}\right)}+C.$$ To determine the constant $C,$ note that with the original equation, we have $y=d$ when $x=0,$ so that we have $$C=-\frac1k \arctan{\left(\frac{3ad}{k}+\frac{b}{k}\right)},$$ so that $$x=\frac1k \arctan{\left(\frac{3ay}{k}+\frac{b}{k}\right)}-\frac1k \arctan{\left(\frac{3ad}{k}+\frac{b}{k}\right)}.$$

It is now an easy matter to find the real root $x$ of the original equation, which is the value of $x$ when $y=0.$ This gives the number $$x=\frac1k \arctan{\left(\frac{b}{k}\right)}-\frac1k \arctan{\left(\frac{3ad}{k}+\frac{b}{k}\right)}.$$

Now consider applying this procedure to the example $$2x^3+3x^2+2x+3=0.$$ Clearly $3^2-3(2)(2)<0,$ so it is of the required type. Also, it is easy to see that it has only one real root, namely $x=-3/2.$ However, applying the method above, we obtain $k=\sqrt 3,$ so that the root as given by the arctangent should be $$x=\frac {1}{\sqrt 3} \arctan{\left(\frac{3}{\sqrt 3}\right)}-\frac{1}{\sqrt 3} \arctan{\left(\frac{21}{\sqrt 3}\right)}.$$ If this is so, then we must have $$\frac{1}{\sqrt 3} \arctan{\left(\frac{3}{\sqrt 3}\right)}-\frac{1}{\sqrt 3} \arctan{\left(\frac{21}{\sqrt 3}\right)}=-\frac 3 2,$$ which gives $$\arctan{(7\sqrt 3)}-\arctan{\sqrt 3}=\frac{3\sqrt 3}{2},$$ which is clearly false.

I have checked again and again, but have failed to see where I went wrong. Please help me spot the false step. Many thanks!

PS. This method works seamlessly well with equations of first order, as can be easily checked; so I kept wondering where the analogy breaks down.