Suppose that is a continuously differentiable function such that at every point of the plane at least one of the partial derivatives is zero.

Prove that depends on just one variable (either or ). In other words, either or .

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# Category: Mathematics

## Rigidity of functions

## Controlled bilipschitz extension

## Almost norming functionals, Part 1

## Expanding distances on a sphere

## Generalized Birthday Problem

Suppose that is a continuously differentiable function such that at every point of the plane at least one of the partial derivatives is zero.

Prove that depends on just one variable (either or ). In other words, either or .

A map is -bilipschitz if for all . This definition makes sense if X and Y are general metric spaces, but let’s suppose they are subsets on the plane .

Definition 1. A set has *the BL extension property* if any bilipschitz map can be extended to a bilipschitz map . (Extension means that is required to agree with on .)

Lines and circles have the BL extension property. This was proved in early 1980s independently by Tukia, Jerison and Kenig, and Latfullin.

Definition 2. A set has *the controlled BL extension property* if there exists a constant such that any -bilipschitz map can be extended to a -bilipschitz map .

Clearly, Definition 2 asks for more than Definition 1. I can prove that a line has the controlled BL extension property, even with a modest constant such as . (Incidentally, one cannot take .) I still can’t prove the controlled BL extension property for a circle.

**Update:** extension from line is done in this paper.

Let be a real Banach space with the dual . By the Hahn-Banach theorem, for every unit vector there exists a functional of unit norm such that . One says that is a norming functional for . In general, one cannot choose so that it depends continuously on . For example, the 2-dimensional space with norm does not allow such a continuous selection.

Fix and call a linear functional *almost norming* for if and . In any Banach space there exists a continuous selection of almost norming functionals.

This came up in a discussion at AIM.

Let be the unit sphere. Suppose that is a continuous map which does not decrease distances: that is, for all . By the generalized Jordan theorem the complement of has two components, precisely one of which, denoted , is bounded.

Prove that contains an open ball of radius 1.

Facebook currently allows up to 5000 friends. Assume that birthdays are uniformly distributed among 365 days. It is well-known that for a user with 23 friends there is a 50% chance of having to write ”Happy Birthday” on at least two walls in the same day.

How many facebook friends should one acquire to have a 50% chance of at least 10 birthdays falling on the same day? With 3286 friends this is guaranteed to happen, but one should expect that 50% probability will be reached with a substantially smaller number.

**[Update]** In 1989 Diaconis and Mosteller found an approximate solution that gives an approximate number of people required to have at least 50% probability of of at least birthdays on the same day. The formula can be found on MathWorld and its output is the OEIS sequence A050255. For comparison, Diaconis and Mosteller quote exact values found by Bruce Levin. The exact solution is the sequence A014088, which begins thus:

1, 23, 88, 187, 313, 460, 623, 798, 985, 1181, 1385, 1596, 1813, 2035, 2263

So, with 1181 Facebook friends there is a 50% chance of having to write “Happy Birthday” at least ten times in the same day.