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I was reading the thesis http://www.renyi.hu/~keszegh/papers/szakdolgozat.pdf, and they cite Theorem 2.6 without proof (because apparently it is a well-known fact from extremal graph theory).

The extremal function $\text{ex}(n, P)$ is defined in the paper as the maximum possible number of 1's in an $n\times n$ binary matrix that avoids a pattern $P$.

Prove that $\text{ex}\left(n, \begin{pmatrix} 1 & 1\\ 1 & 1\end{pmatrix}\right) = \Theta(n^{3/2})$.

Can someone post a proof here or redirect me to one? Thanks!

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You might try:

Corollary 2.4 in "On 0-1 matrices and small excluded submatrices" (.ps)

What occurs in the above is a citation to Theorem 1.1 in "Davenport-Schinzel theory of matrices" (.pdf), which is the actual proof.

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    I'm looking for a proof of the theorem. Theorem 1.1 is just stated as a "classical result" without proof, so that doesn't really help me. – 2017-02-11
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    The references cited in "Davenport-Schinzel theory of matrices" could provide a proof for that specific statement. In this case, it seems that finding approved is more a case of searching the literature than having someone provide one. That said, particular statement seems to be a result of the Kővári-Sós-Turán (KST) theorem, which provides an upper, sharp bound for extremal graphs and matrices--a proof of which can be found here: http://math.mit.edu/~cb_lee/18.318/lecture2.pdf – 2017-02-12