Erdős Problem #426

Erdős Problem 426

Statement and result.

Can an $n$-vertex graph have a positive constant multiple of $2^{\binom n2}/n!$ nonisomorphic subgraphs which each occur exactly once? The answer is no. We reconstruct the qualitative argument of Bradač and Christoph. A subgraph here need not be induced. Put $N=\binom n2$ and let $f(H)$ denote the number in question, divided by $2^N/n!$. We prove uniformly that $f(H)=o(1)$.

Standard external inputs.

We use the graph-enumeration theorem $U_n\sim2^N/n!$, where $U_n$ counts unlabelled $n$-vertex graphs (Pólya--Wright), elementary binomial concentration and the bound $\max_m\mathbb P(\operatorname{Bin}(N,1/2)=m)=O(1/n)$, and the bounded-differences inequality \[ \mathbb P(Z\leq\mathbb EZ-t) \leq\exp\left(-\frac{2t^2}{\sum_i c_i^2}\right) \tag{1} \] for a function of independent variables with coordinate influences at most $c_i$. These inputs are not reproved here.

1. Passing from isomorphism types to random embeddings.

The identity \[ 2^N=\sum_{F\text{ unlabelled},\,|V(F)|=n} \frac{n!}{|\operatorname{Aut}(F)|} \] and the enumeration theorem imply that the number of types with a nontrivial automorphism is $o(2^N/n!)$: each such type accounts for a deficit at least $n!/2$ in comparison with $n!U_n$. Also $\sum_{k<n}U_k=o(2^N/n!)$. To see this last assertion, for $k\geq n/2$ use $U_k\leq C2^{\binom k2}/k!$; consecutive terms have ratio $k2^{-(k-1)}$, so the sum is dominated by its last term. For $k<n/2$, the crude bound $U_k\leq2^{\binom k2}$ suffices.

An asymmetric spanning subgraph has one copy in $H$ exactly when there is one vertex bijection embedding it into $H$. Each asymmetric type has $n!$ labellings. It follows, uniformly in $H$, that \[ f(H)=\mathbb P_{G\sim G(n,1/2)} (G\text{ has exactly one embedding into }H)+o(1). \tag{2} \]

2. Positive probability forces only $O(n)$ missing edges.

Suppose the probability in $2$ is at least a fixed $\delta>0$. Choose $L=L(\delta)$ so that the binomial edge count lies in $I=[N/2-Ln,N/2+Ln]$ except with probability at most $\delta/2$. Consider the random graph process $G_0\subset\cdots\subset G_N$, adding a uniformly random unused edge each time. Let $X$ count the integers $m\in I$ at which $G_m$ has exactly one embedding into $H$. Conditional on its edge count, $G(n,1/2)$ has the distribution of $G_m$. The binomial point-mass bound gives $\mathbb EX\geq c_\delta n$. Since $X\leq2Ln+2$, there are constants $\alpha,\beta>0$, depending only on $\delta$, such that \[ \mathbb P(X\geq\alpha n)\geq\beta. \tag{3} \]

The number of embeddings is nonincreasing as edges are added, so the indices with exactly one embedding form an interval. Stop at the first such index in $I$, when one exists. By conditioning on the stopped history, $3$ supplies a history for which the conditional probability of $X\geq\alpha n$ is at least $\beta$. Write $m$ for that index and $\pi$ for its unique embedding. For $h=\lfloor\alpha n\rfloor-1$, the event in $3$ requires that $\pi$ survive the next $h$ edges. The random process after the stopping time still adds edges uniformly, so this probability is \[ \frac{\binom{e(H)-m}{h}}{\binom{N-m}{h}} \leq\left(\frac{e(H)}N\right)^h. \tag{4} \] If the numerator has too few available edges, its value is zero. Writing $t=N-e(H)$, $4$ is at most $\exp(-th/N)$. Thus $\beta\leq\exp(-th/N)$, which implies $t\leq C_\delta n$.

3. A sparse complement supplies many possible switches.

Fix $C\geq1$, assume the complement $K=\overline H$ has at most $Cn$ edges, and let $J\sim G(n,1/2)$. An embedding $G\to H$ corresponds, by inversion, to an embedding $K\to\overline G$; thus it suffices to exclude a unique embedding $K\to J$ with probability $1-o(1)$.

At least $n/2$ vertices of $K$ have degree less than $4C$. The graph on these vertices has an independent set $B$ of size at least $n/[2(4C+1)]$, by the greedy independent-set bound. For large $n$, start with $T=B$, $D=1$, so $|T|\geq n/(\log n)^D$. If an outside vertex has a neighborhood in $T$ which is not all of $T$, but has size at least $n/(\log n)^{6D}$, replace $T$ by that neighborhood and replace $D$ by $6D$. Each chosen outside vertex becomes adjacent to all later members of $T$; chosen vertices are distinct. Since every member of $T$ has degree less than $4C$, fewer than $4C$ refinements are possible. The final $D$ is bounded in terms of $C$, $T$ is independent, and, writing $d_w=|N_K(w)\cap T|$, every $w\notin T$ has either \[ d_w=|T|\quad\hbox{or}\quad d_w<n/(\log n)^{6D}. \tag{5} \]

Fix any bijection $\pi:V(K)\to V(J)$. For $u,v\in T$, call the pair switchable if $J$ contains all edges from $\pi(u)$ to $\pi(N_K(v)\setminus N_K(u))$ and from $\pi(v)$ to $\pi(N_K(u)\setminus N_K(v))$. If $\pi$ is already an embedding, switching its values at $u,v$ then gives a different embedding. Common neighbors need no new edges, and $u,v$ are nonadjacent in $K$.

Let $Z$ count switchable unordered pairs from $T$. Each event requires at most $8C$ independent edges, so \[ \mathbb EZ\geq2^{-8C}\binom{|T|}{2}. \tag{6} \] Only edges of $J$ corresponding to pairs $uw$, $u\in T,w\notin T$, can influence $Z$. When $w$ is adjacent to all of $T$, such an edge has no influence. Otherwise its influence is at most $d_w$. Using $5$ and $\sum_wd_w\leq2Cn$, \[ \sum_i c_i^2 \leq |T|\sum_{\substack{w\notin T\\d_w<|T|}}d_w^2 \leq\frac{2Cn^3}{(\log n)^{6D}}. \tag{7} \] Equations $1$, $6$, $7$, and $|T|\geq n/(\log n)^D$ give \[ \mathbb P(Z=0) \leq\exp\left(-c_C n(\log n)^{2D}\right) =o(1/n!), \tag{8} \] uniformly over $\pi$. A union bound over all $n!$ bijections says that, with probability $1-o(1)$, every embedding of $K$ into $J$ can be switched to a second one. There is therefore no unique embedding. If every influence in $7$ is zero, $Z$ is deterministic and positive by $6$, so the same conclusion holds without dividing by zero in $1$.

Conclusion and attribution.

If $f(H)$ failed to tend to zero uniformly, some fixed positive $\delta$ and graphs on an unbounded sequence of orders would contradict $2$, Step 2, and Step 3. Thus the answer is negative. This reconstructs the qualitative proof of D. Bradač and M. Christoph, Unique subgraphs are rare, https://arxiv.org/abs/2410.16233. Their stronger quantitative rate is not asserted here. Statement checked at https://www.erdosproblems.com/426 on 24 September 2026. The explicit standard inputs above remain external.

Completion estimate.

The qualitative disproof is complete using the stated enumeration and concentration theorems. COMPLETION: 100%