数学符号π (Pi)、Σ(Sigma)的来历

一、π (Pi)

March 14 marks Pi Day, the holiday commemorating the mathematical constant π (pi), written numerically as 3.141592+. Since mathematic notation is a language that uses symbols from a multitude of alphabets and typefaces, it seems only fitting that this sixteenth letter of the Greek alphabet get a little attention.

3月14日是Pi节，这个节日纪念数学常量π (pi)，即在数学上等于3.141592+的数。

The Latin name of the Greek letter π is pi, pronounced pie. The first recorded use of the letter as a mathematical symbol comes from the Welsh mathematician William Jones in a 1706 work called Synopsis Palmariorum in which he abbreviated periphery or its Greek ancestor περι??ρεια, meaning “circumference,” to π.

Greek(希腊)字母π在Latin（拉丁语）中转写成了pi,发音类似pie（派）。首次是一个Welsh（威尔士）数学家William Jones在1706年的《Synopsis Palmariorum》书中使用，在这本书中William Jones将periphery或者其希腊词源 περι??ρεια(指‘circumference‘「周长」)缩写成了π。

The mathematical pi is defined as the ratio of the circumference of a circle to its diameter. As an irrational number, pi can never repeat when written in decimals; computers have calculated pi to decimal places in the trillions. It is also a transcendental number, a concept that exceeds the capacity of this post to explain.

数学上的pi被定义成圆形的周长和其直径的比。作为一个irrational number(无理数，不能表示成两个整数相除的分数形式)，pi如果用十进制记录，pi是不会重复的（不循环）。

Memorizing as many digits of pi as possible is an obsession for some. The Guinness World Record for memorizing digits of π is held by a man named Lu Chao. He set the record in November 2005 at Northwest A&F University in the Shaanxi province of China. It took him 24 hours and 4 minutes to recite the 67,890th decimal place of π without a mistake.

1.  参考：What’s the Origin of the Word “Pi”?

二、Σ(Capital Sigma)

Σ(Sigma) is the upper case letter S in Greek. And S stands for Sum(和).

参考：Sigma Notation

三、μ(Mu)

μ (Mu) is mean(均值、平均).

四、σ(sigma)

五、

∏(capital pi)

product - product of all values in range of series(一系列书的乘积) ∏ x_i=x₁?x₂?...?x_n.