Proof that steady state is not affected by initial distribution in Markov chain. I was following a proof provided in Gilbert Strang's book "Introduction to Linear Algebra".
And I am confused by one step of the proof.
Suppose we have a $n$ by $n$ stochastic matrix $A$, where all elements are not negative and sum of every element in each row is $1$.
There exists a proof that there is only one biggest eigenvalue of $A$ equal to $1$ and other eigenvalues are less then $1$.
I found it there:
Proof that the largest eigenvalue of a stochastic matrix is 1
Now I want to get a proof that Markov chain has a steady state that is not affected by initial probability distribution:
$$ u_0 = \left( \begin{smallmatrix} u_1 \\ u_2 \\ \ldots \\ u_n \end{smallmatrix} \right) $$
Let's apply diagonalization to our matrix $A$:
$$ A = S \Lambda S^{-1} $$
Where $S$ consists of eigenvectors placed there as columns,
$\Lambda$ is a diagonal matrix with corresponding eigenvalues.
Suppose we want to get a presentation of our initial distribution as a linear combination of eigenvectors of $A$ like this:
$$ u_0 = c_1 x_1 + c_2x_2 + \ldots + c_nx_n $$
In a matrix form:
$$u_o = SC$$
We can get $C$ from:
$$C = S^{-1}u_0$$
So, when we apply A to $u_0$ multiple times ($k$ times):
$$ u_k = A \ldots Au_o = S \Lambda S^{-1} \ldots S \Lambda S^{-1} u_o = S \Lambda S^{-1} \ldots S \Lambda S^{-1} SC = S \Lambda^{k} C$$
So, literally we get:
$$ u_k = c_1(\lambda_1)^kx_1 + c_2(\lambda_2)^kx_2 + \ldots + c_n(\lambda_n)^kx_n $$
But then author writes this in the book:
$$ u_k = x_1 + c_2(\lambda_2)^kx_2 + \ldots + c_n(\lambda_n)^kx_n $$
I understand that the author omits $\lambda_1$ because it is equal to 1.
Why does the author omit $c_1$?
EDIT: I found out that $c_1$ is equal to 1. But I don't know why.
This is why the author omits it.
Later in his proof the author shows that:
$$ \lim_{k\rightarrow \infty } u_k = x_1 $$
So, the author concludes that steady state is equal to the eigenvector with corresponding eigenvalue of 1.

There is something wrong in your question, $c_1=1$ is a special case for the example matrix $\begin{bmatrix}.80&.05\\.20&.95\end{bmatrix}$ in Strang's book. But assume that $\lambda_1=1$, the corresponding $c_i$ must be a CONSTANT for any initial state $u_0$, like 1 of $\begin{bmatrix}.80&.05\\.20&.95\end{bmatrix}$.
Proof
The answer of 'user940' is almost right, but he made a little mistake in the last step, maybe misled by the question.
The right last step is
$$
\begin{aligned}
1^T u_0 &= 1^T c_1 x_1 + 1^T c_2 x_2 + \dots + 1^T c_n x_n \\&=1^Tc_1x_1 \quad (\text{ for } 1^T x_j=0 \text{ if } j\neq1)
\end{aligned}
$$
so,
$$
\begin{aligned}
c_1&=\frac{1^Tu_0}{1^Tx_1} \\
&=\frac{1}{1^Tx_1} \quad (\text{ for } u_0 \text{ is a probability distribution })
\end{aligned}
$$
So, for any initial state $u_0$, the steady state is $$
\lim_{x \to \infty}{u_k}= c_1 x_1 = \frac{x_1}{1^T x_1}
$$
One word more, in the example matrix $\begin{bmatrix}.80&.05\\.20&.95\end{bmatrix}$ in Strang's book. $x_1=\begin{bmatrix} .2 \\ .8 \end{bmatrix}$, so $c_1=1$
