Tivadar Danka Profile picture
May 11, 2021 β€’ 9 tweets β€’ 3 min read β€’ Read on X
There is a mathematical formula so beautiful that it is almost unbelievable.

Euler's identity combines the famous numbers 𝑒, 𝑖, Ο€, 0, and 1 in a single constellation. At first sight, most people doubt that it is true. Surprisingly, it is.

This is why.

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Let's talk about the famous exponential function 𝑒ˣ first.

Have you ever thought about how is this calculated in practice? After all, raising an irrational number to any power is not trivial.

It turns out that the function can be written as an infinite sum!
In fact, this can be done with many other functions.

For those that are differentiable infinitely many times, there is a recipe to find the infinite sum form. This form is called the Taylor expansion.

It does not always yield the original function, but it works for 𝑒ˣ.
Taylor expansions are advantageous for two reasons.

First, we can approximate functions by cutting of the sum at some N.

Second, we can simply extend functions to the complex plane with this formula!
The exponential function is not the only one that can be written as a Taylor series.

We can also do this with the trigonometric functions sine and cosine.

(Feel free to check this by hand using the general Taylor expansion formula.)
By plugging in 𝑖𝑧 into the exponential function, we discover that the complex exponential function can be written in terms of trigonometric functions!

(We use that 𝑖² = -1.)
In the special case 𝑧 = Ο€, we obtain the famous formula called Euler's identity.

This is how the magic happens.
When asked, Euler's identity often comes up among mathematicians as the most beautiful formula ever.

It is not only amazing because it connects together a bunch of famous constants, but because it establishes a connection between the exponential and trigonometric functions.
If you enjoyed this explanation, consider following me and hitting a like/retweet on the first tweet of the thread!

I regularly post simple explanations of mathematical concepts in machine learning, make sure you don't miss out on the next one!

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More from @TivadarDanka

Nov 23
The single biggest argument about statistics: is probability frequentist or Bayesian?

It's neither, and I'll explain why.

Buckle up. Deep-dive explanation incoming. Image
First, let's look at what is probability.

Probability quantitatively measures the likelihood of events, like rolling six with a dice. It's a number between zero and one. This is independent of interpretation; it’s a rule set in stone. Image
In the language of probability theory, the events are formalized by sets within an event space.

The event space is also a set, usually denoted by Ξ©.) Image
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Nov 19
The single most undervalued fact of linear algebra: matrices are graphs, and graphs are matrices.

Encoding matrices as graphs is a cheat code, making complex behavior simple to study.

Let me show you how! Image
If you looked at the example above, you probably figured out the rule.

Each row is a node, and each element represents a directed and weighted edge. Edges of zero elements are omitted.

The element in the 𝑖-th row and 𝑗-th column corresponds to an edge going from 𝑖 to 𝑗.
To unwrap the definition a bit, let's check the first row, which corresponds to the edges outgoing from the first node. Image
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Oct 25
The following multiplication method makes everybody wish they had been taught math like this in school.

It's not just a cute visual tool: it illuminates how and why long multiplication works.

Here is the full story: Image
First, the method.

The first operand (21 in our case) is represented by two groups of lines: two lines in the first (1st digit), and one in the second (2nd digit).

One group for each digit.
Similarly, the second operand (32) is encoded with two groups of lines, one for each digit.

These lines are perpendicular to the previous ones.
Read 10 tweets
Oct 21
The way you think about the exponential function is wrong.

Don't think so? I'll convince you. Did you realize that multiplying e by itself Ο€ times doesn't make sense?

Here is what's really behind the most important function of all time: Image
First things first: terminologies.

The expression aᡇ is read "a raised to the power of b."

(Or a to the b in short.) Image
The number a is called the base, and b is called the exponent.

Let's start with the basics: positive integer exponents. By definition, aⁿ is the repeated multiplication of a by itself n times.

Sounds simple enough. Image
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Oct 20
In calculus, going from a single variable to millions of variables is hard.

Understanding the three main types of functions helps make sense of multivariable calculus.

Surprisingly, they share a deep connection. Let's see why: Image
In general, a function assigns elements of one set to another.

This is too abstract for most engineering applications. Let's zoom in a little! Image
As our measurements are often real numbers, we prefer functions that operate on real vectors or scalars.

There are three categories:

1. vector-scalar,
2. vector-vector,
3. and scalar-vector. Image
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Oct 19
The Law of Large Numbers is one of the most frequently misunderstood concepts of probability and statistics.

Just because you lost ten blackjack games in a row, it doesn’t mean that you’ll be more likely to be lucky next time.

What is the law of large numbers, then? Read on: Image
The strength of probability theory lies in its ability to translate complex random phenomena into coin tosses, dice rolls, and other simple experiments.

So, let’s stick with coin tossing.

What will the average number of heads be if we toss a coin, say, a thousand times?
To mathematically formalize this question, we’ll need random variables.

Tossing a fair coin is described by the Bernoulli distribution, so let X₁, Xβ‚‚, … be such independent and identically distributed random variables. Image
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