Blog posts

Teensy Problems: Alien Astronomer

1 minute read

Published: April 27, 2026

Gryxlb, an alien radio astronomer on planet Xorgmathrx, has started intercepting a peculiarly structured signal. There’s a stream of pulses originating from the Orion Arm counting up from zero in binary; it must be a deliberate message from an intelligent civilization! The telescope’s computers are recording the pulses, but one is malfunctioning: instead of writing the pulses into memory, each memory slot is the bitwise exclusive or (xor) of the incoming pulse and the previous memory slot.

Teensy Problems: Kaprekar Numbers

1 minute read

Published: April 10, 2026

Dr. Kaprekar has stumbled upon an interesting class of numbers. A number N is called a “Kaprekar number” if $N^2$, written in base 10, can be split into two parts, $A$ and $B$, such that $N = A + B$. Here are some examples:

   9² =       81 →   8 +     1 =    9
  45² =     2025 →  20 +    25 =   45
4879² = 23804641 → 238 + 04641 = 4879

Notice that the parts do not have to be equal in size and can include leading zeros; the only restriction is that neither part is totally zero. Let’s help Dr. Kaprekar understand more about his numbers.

Teensy Problems: Sum of Factorials

1 minute read

Published: March 13, 2025

Let $\mathrm{sf}(n)$ be the sum-of-factorials function: $0! + 1! + … n!$. Its first few values are $1$, $2$, $4$, $10$, $34$, ….

Teensy Problems: Falling Leaves

1 minute read

Published: November 24, 2024

Autumn and Yvonne love to play “falling leaves” this time of year. They play on a tree containing some number of leaves (possibly none) on its left and right sides. Starting with Autumn, each player takes turn choosing a leaf. All leaves at or below the chosen leaf on the same side of the tree fall to the ground. The game ends once the tree is out of leaves; whoever is the last to choose a leaf is the winner.

Teensy Problems: Coprime Labeling

1 minute read

Published: October 26, 2024

Let $G$ be a graph with $n$ vertices. We say a graph is coprimely labeled if its vertices are labeled using all the integers from 1 to $n$ in such a way where, if any two vertices have an edge between them, their labels are coprime; that is, they share no common factors.

Teensy Problems: Marvin Numbers

less than 1 minute read

Published: October 25, 2024

Marvin is interested in numbers whose sum of their digits equals the product of their digits. Call a nonnegative integer a $b$-Marvin number if it has this property when written in base $b$. Marvin noticed already that all single digits in base $b$ are trivially $b$-Marvin numbers.

Teensy Problems: Counting Carol

less than 1 minute read

Published: October 04, 2024

Carol really likes counting, but can only keep track of so many numbers at once. She’s devised a family of sequences that matches these aptitudes, in the following manner: Let $A_k(1) = 1$, then let $A_k(n+1)$ be the number of times $A_k(n)$ appears in the last $k$ terms of $A_k$.

Teensy Problems: Finn Factor

1 minute read

Published: September 18, 2024

Finn Factor just learned about a neat way to compute the prime numbers. Let $S$ be the list of known primes (initially empty), and start at $n = 2$. For each $n$, we ask if it is divisible by any elements of $S$. If it isn’t, we add it to $S$; otherwise, $n$ is composite. We increment $n$ and continue, which will place into $S$ every prime up until wherever we decide to stop.