Consider the following sets.
$S_1=\{a_1n^2+b_1n+c_2|n\in \mathbb{Z}\}$
$S_2=\{a_2n^2+b_2n+c_2|n\in \mathbb{Z}\}$
.....
$S_k=\{a_kn^2+b_kn+c_k|n\in \mathbb{Z}\}$
Where, $a_1,a_2,...,a_k\not = 0$

Prove that,
$\bigcup_{j=1}^k S_j\not = \mathbb{Z}$
For any $k\geq 1$
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What I am trying to show, informally, that a set of integers cannot be placed in classes of quadradic progressions.

For example, with linear progressions it is always possible. Consider
$3k,3k+1,3k+2$-they contain all the numbers.

2. Consider the number of integers less than X which are represented by the form $an^2 + bn + c$. It is not too hard to show that this number is approximately $\sqrt{X/a}$. So the number of integers represented by one of k such forms is at most a quantity approximately $\sqrt{X} \sum_{i=1}^k \frac1{\sqrt{a_i}}$ and for large enough X this must be less than X. Hence some numbers are not so represented.