Disproved by Janzer [Ja21] who constructed, for any $\epsilon>0$, a $3$-regular bipartite graph $H$ such that \[\mathrm{ex}(n;H)\ll n^{\frac{4}{3}+\epsilon}.\]
See also [146] and [147] and the entry in the graphs problem collection.
Is it true that, for every $\mathcal{F}$, there exists $G\in\mathcal{F}$ such that \[\mathrm{ex}(n;G)\ll_{\mathcal{F}}\mathrm{ex}(n;\mathcal{F})?\]
This is trivially true if $\mathcal{F}$ does not contain any bipartite graphs, since by the Erdős-Stone theorem if $H\in\mathcal{F}$ has minimal chromatic number $r\geq 2$ then \[\mathrm{ex}(n;H)=\mathrm{ex}(n;\mathcal{F})=\left(\frac{r-2}{r-1}+o(1)\right)\binom{n}{2}.\] Erdős and Simonovits observe that this is false for infinite families $\mathcal{F}$, e.g. the family of all cycles.
See also [575] and the entry in the graphs problem collection.
See also [712] for the general case.
A rational $\alpha\in [1,2]$ for which the first problem holds is known as a Turán exponent. Known Turán exponents are:
See also [713] and the entry in the graphs problem collection.
Erdős [Er64c] and Bondy and Simonovits [BoSi74] showed that \[\mathrm{ex}(n;C_{2k})\ll kn^{1+\frac{1}{k}}.\]
Benson [Be66] has proved this conjecture for $k=3$ and $k=5$. Lazebnik, Ustimenko, and Woldar [LUW95] have shown that, for arbitrary $k\geq 3$, \[\mathrm{ex}(n;C_{2k})\gg n^{1+\frac{2}{3k-3+\nu}},\] where $\nu=0$ if $k$ is odd and $\nu=1$ if $k$ is even. See [LUW99] for further history and references.
See also [765] and the entry in the graphs problem collection.
See also [574] and the entry in the graphs problem collection.
See also [573] and the entry in the graphs problem collection.
Is it true that, for every $\mathcal{F}$, if there is a bipartite graph in $\mathcal{F}$ then there exists some bipartite $G\in\mathcal{F}$ such that \[\mathrm{ex}(n;G)\ll_{\mathcal{F}}\mathrm{ex}(n;\mathcal{F})?\]
See also [180] and the entry in the graphs problem collection.
A theorem of Sudakov and Tomon [SuTo22] implies \[\mathrm{ex}(n;Q_k)=o(n^{2-\frac{1}{k}}).\] Janzer and Sudakov [JaSu22b] have improved this to \[\mathrm{ex}(n;Q_k)\ll_k n^{2-\frac{1}{k-1}+\frac{1}{(k-1)2^{k-1}}}.\] See also the entry in the graphs problem collection.
See also [533] and the entry in the graphs problem collection.
See also [500] for the case $r=3$ and $k=4$.
See also [571].
See also [572].
Give good estimates for $f(n;k,l)$ in the range $k<l\leq k^2/4$. For fixed $k$ and large $n$ is $f(n;k,l)$ a strictly monotone function of $l$?
The conjectured equality was proved for $n\geq 3k+3$ by Jiang [Ji04].
Curiously, in [Er69b] Erdős mentions this problem, but states that his conjectured equality for $g_k(n)$ was disproved (for general $k$) by Lewin, citing oral communication. Perhaps Lewin only disproved this for small $n$, or perhaps Lewin's disproof was simply incorrect.
This problem is then now asking how many edges a $3$-uniform hypergraph can have before it contains $K_4$ minus an edge, and whether the critical edge density is $2/9$. In fact there is a construction of Frankl and Füredi [FrFu84] showing it must be at least $2/7$, which is the conjectured truth (although Turán conjectured before [Er69] that that the edge density was $1/4$, and so likely there is simply a typo in this problem's statement).
Harris has provided the following simple counterexample to the problem as stated: the $3$-uniform graph on $\{1,\ldots,9\}$ with $28$ edges, formed by taking $27$ edges by choosing one element each from $\{1,2,3\},\{4,5,6\},\{7,8,9\}$, and then adding the edge $\{1,2,3\}$.