Trefoil to Figure-Eight by Crossing Changes

A knot is a simple closed curve in three-dimensional space. A diagram of a knot is a projection of the knot to the plane together with over/under information at each crossing. A crossing change is the operation of switching one crossing from over to under or from under to over.

Find up to three examples of sequences of knot diagrams that begin with a diagram of the trefoil knot and end with a diagram of the figure-eight knot, where each step is obtained from the previous diagram by a crossing change.

The obvious chains trefoil → unknot → figure-eight and trefoil → trefoil # figure-eight → figure-eight are not allowed.

(3 points will be given for three correct examples, 2 points for two correct examples, and 1 point for one correct example.)

Let
\[
Q_n=\{0,1\}^n
\]
be the \(n\)-dimensional discrete cube, viewed as a graph in which two vertices are adjacent if they differ in exactly one coordinate.

For a subset \(A\subseteq Q_n\), let \(\partial_e A\) denote the set of edges with one endpoint in \(A\) and the other in \(Q_n\setminus A\).

Prove that for every \(A\subseteq Q_n\),
\[
|\partial_e A|\ge |A|\bigl(n-\log_2|A|\bigr).
\]

Let \(f(x)\) be a function such that \((1-x^2) f”(x) – 2x f'(x) + \alpha (\alpha+1) f(x) =0\)
for some \(\alpha \not\in \mathbb{N}\). Define \(P_n (x) = \frac{d^n}{dx^n} (x^2-1)^n\) for \(n =0,1,…\). Compute \(\int_{-1}^1 f(x) P_n(x) dx.\)

A link in S³ is a smooth embedding of a finite disjoint union of circles into S³. A link diagram is a generic projection to S² together with over/under data at each double point. For an oriented 2-component link K ∪ J, the linking number lk(K, J) is one-half of the signed sum of the crossings between K and J.

Prove or disprove that if lk(K, J) = 0, then there exist disjoint, compact, properly embedded, orientable surfaces F₁, F₂ ⊂ S³ × I such that

∂F₁ = K × {1}
∂F₂ = J × {1}.

Your solution should consist almost entirely of pictures. Each picture may have at most one short explanatory sentence.

(It turns out that the converse is also true.)

Let \(n\) be an odd positive integer, and let
\[
f:\{-1,1\}^n\to\{-1,1\}.
\]
Interpret \(x_i=1\) as voter \(i\) voting for candidate \(A\), and \(x_i=-1\) as voter \(i\) voting for candidate \(B\). The value \(f(x_1,\dots,x_n)\) is the choice.

Find all functions \(f\) satisfying the following properties:
1. Anonymity: for every permutation \(\sigma\in S_n\),
\[
f(x_1,\dots,x_n)=f(x_{\sigma(1)},\dots,x_{\sigma(n)}).
\]
2. Neutrality:
\[
f(-x_1,\dots,-x_n)=-f(x_1,\dots,x_n).
\]
3. Monotonicity: if \(x=(x_1,\dots,x_n)\) and \(y=(y_1,\dots,y_n)\) satisfy
\[
x_i\le y_i \qquad \text{for all } i=1,\dots,n,
\]
then
\[
f(x)\le f(y).
\]

Let \(V\) be the set of tuples \((a_1,…,a_5)\) such that \(a_1 \leq a_2 \leq \cdots \leq a_5 \) belong to \(\mathbb{R}\) and satisfy \[ \sum_{1\leq i\leq 5} a_i >0, \quad \sum_{1\leq i<j \leq 5} a_i a_j >0, \quad \sum_{1\leq i<j< k \leq 5} a_ia_ja_k >0.\]

What is the maximum number \(p\) such that there exists a tuple \((a_1,…,a_5) \) in \(V\) whose \(a_p\leq 0 \)?

The email account pow@mathsci.kaist.ac.kr was not working normally. It is now fixed.

POW2026-01 is revised to clarify the problem. (The revision is only for the clarification and there is essentially no change in the problem.)

Prove that for every positive integer \( k \) there exists a positive integer \( n \) such that
\[
\frac{(n+1)(n+2) \dots (2n-k)}{n(n-1) \dots (n-k+1)}
\]
is an integer and that \( k = o(n) \) for such \( n \).