Absolute value of a Maclaurin series I am trying to find out the absolute value of a Maclaurin power series of the below type:
$f(x) = a_0 + a_1 x+a_2 x^2+ \dots + a_n x^n$, where $x$ is a complex number. I am interested to know the vakue of $|f(x)| = |a_0 + a_1 x+a_2 x^2+ \dots + a_n x^n|$. 
I have tried looking around and found something relevant here. But, I am not sure if this relates to my problem and also what is the name of the equation $|\lambda(z)| = 1 + Re(a_n z^n) + O(z^n)$ and meaning of the term $O(z^n)$. Please let me know if there is any guidance in this regard. Thanks!
 A: Clarification
The Maclaurin series of a function $f(z)$ is a special case of the Taylor series...
$$T_f^{z_0}(z) = \sum_{k=0}^{\infty}{\frac{f^{(k)}(z_0)}{n!}(z - z_0)^k}$$
Where $z_0 = 0$...
$$T_f^{0}(z) = \sum_{k=0}^{\infty}{\frac{f^{(k)}(0)}{k!}z^k}$$
It is important to make the correction that $f(z)$ does not equal, it approximates the finite sequence...
$$f(z) \approx a_0 + a_1z + a_2z^2 + \ldots + a_nz^n$$
It is equal to the infinite sum...
$$f(z) = a_0 + a_1z + a_2z^2 + \ldots + a_nz^n + \ldots$$
So, the absolute value of $f(z)$ would be...
$$\left|T_f^{0}(z)\right| = \left|\sum_{k=0}^{\infty}{\frac{f^{(k)}(0)}{k!}z^k}\right|$$

Example
What is the Maclaurin series for $g(z) = e^z$?
$$
T_g^{0}(z) = \sum_{k=0}^{\infty}{\frac{g^{(k)}(0)}{k!}z^k}
\\\implies e^z = \sum_{k=0}^{\infty}{\frac{z^k}{k!}}
$$
What is the Maclaurin series for $f(z) = \left|g(z)\right|$?
$$
T_f^{0}(z) = \left|\sum_{k=0}^{\infty}{\frac{g^{(k)}(0)}{k!}z^k}\right|
\\\implies\left|e^z\right| = \left|e^{\operatorname{Re}(z) + i\operatorname{Im}(z)}\right| = \left|e^{\operatorname{Re}(z)}\right|\left|e^{i\operatorname{Im}(z)}\right| = e^{\operatorname{Re}(z)}
\\\implies\left|e^z\right| = \sum_{k=0}^{\infty}{\frac{\operatorname{Re}(z)^k}{k!}}
$$
