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(This is a follow-up to thisthis question.)

Let $X\neq \emptyset$ be a finite set and suppose that ${\cal C}$ is a set of subsets of $X$ with the following properties:

  1. all members of ${\cal C}$ contain at least $2$ elements, and $X\notin {\cal C}$;
  2. $A\neq B\in {\cal C}$ implies $|A\cap B| \leq 1$; and
  3. for all $x,y\in X$ there is $A\in {\cal C}$ such that $\{x,y\}\subseteq A$.

Let $m=|{\cal C}|$. Is there a bijection $f: \{1,\ldots, m\}\to {\cal C}$ such that for all $k \in \{1,\ldots, m-1\}$ we have $f(k)\cap f(k+1)\neq \emptyset$?

(This is a follow-up to this question.)

Let $X\neq \emptyset$ be a finite set and suppose that ${\cal C}$ is a set of subsets of $X$ with the following properties:

  1. all members of ${\cal C}$ contain at least $2$ elements, and $X\notin {\cal C}$;
  2. $A\neq B\in {\cal C}$ implies $|A\cap B| \leq 1$; and
  3. for all $x,y\in X$ there is $A\in {\cal C}$ such that $\{x,y\}\subseteq A$.

Let $m=|{\cal C}|$. Is there a bijection $f: \{1,\ldots, m\}\to {\cal C}$ such that for all $k \in \{1,\ldots, m-1\}$ we have $f(k)\cap f(k+1)\neq \emptyset$?

(This is a follow-up to this question.)

Let $X\neq \emptyset$ be a finite set and suppose that ${\cal C}$ is a set of subsets of $X$ with the following properties:

  1. all members of ${\cal C}$ contain at least $2$ elements, and $X\notin {\cal C}$;
  2. $A\neq B\in {\cal C}$ implies $|A\cap B| \leq 1$; and
  3. for all $x,y\in X$ there is $A\in {\cal C}$ such that $\{x,y\}\subseteq A$.

Let $m=|{\cal C}|$. Is there a bijection $f: \{1,\ldots, m\}\to {\cal C}$ such that for all $k \in \{1,\ldots, m-1\}$ we have $f(k)\cap f(k+1)\neq \emptyset$?

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A walk through a set of sets

(This is a follow-up to this question.)

Let $X\neq \emptyset$ be a finite set and suppose that ${\cal C}$ is a set of subsets of $X$ with the following properties:

  1. all members of ${\cal C}$ contain at least $2$ elements, and $X\notin {\cal C}$;
  2. $A\neq B\in {\cal C}$ implies $|A\cap B| \leq 1$; and
  3. for all $x,y\in X$ there is $A\in {\cal C}$ such that $\{x,y\}\subseteq A$.

Let $m=|{\cal C}|$. Is there a bijection $f: \{1,\ldots, m\}\to {\cal C}$ such that for all $k \in \{1,\ldots, m-1\}$ we have $f(k)\cap f(k+1)\neq \emptyset$?