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# Tag Archives: Math

# Telephone Numbers

#### Telephone Numbers

Relevant link: http://en.wikipedia.org/wiki/Telephone_number_(mathematics)

Also called the **involution numbers**, they can be determined by the recurrence:

a_0 = a_1 = 1;

a_n = a_{n−1} + (n − 1) a_{n−2}, for n > 1.

More interesting, they also count the number of involutions on a set with n elements — note that an **involutary function**, is a function f that is its own inverse, that is,

f(f(x)) = x for all x in the domain of f.

(Via Spiked Math.)

# Discrete Mathematics with Ducks

# The Month of the Flexagon

# Arithmetic for Beginners

# Hash Tables

# Tau vs Pi

#### MFT – Tau

(Via Spiked Math.)

# .999… = 1 , 9.999… reasons in 9.999… minutes

# Mathy Limericks (comic)

# The math checks out

(Via Math Fail.)

# 3/4 and Kleiber’s Law (video)

3/4 and Kleiber’s Law (via Numberphile)

Neat video, especially if you like biology as well as math!

(Via Math Fail.)

# Taking math to a whole new level

# Angle-a-trons

# Not a Pineapple under the Sea

#### Open Letter to Nickelodeon, Re: SpongeBob’s Pineapple under the Sea

SpongeBob’s supposed “Pineapple under the Sea,” exposed as the mathematical impossibility it really is:

(Via Vi Hart.)

# Spirals, Fibonacci, and Being a Plant, Part 3

# Spirals, Fibonacci, and Being a Plant, Part 2!

#### Spirals, Fibonacci, and Being a Plant, Part 2!

Part 1: http://youtu.be/ahXIMUkSXX0

Stay tuned for part 3, where we solve ALL the mysteries. Well, sort of. Coming soon! (2 weeks?)

(Via Vi Hart.)