What are well-known constants?

"ISO 80000-2 Quantities and Units - Mathematics" provides a list of well-known constants. Here are some examples:

π - π (pi, U-03C0, π or π) is defined as the ratio of a circle's circumference to its diameter:

<math><mi mathvariant="normal">π</mi><mo>≝</mo>
  <mfrac><mi>c</mi><mi>d</mi></mfrac></math>
    $\mathrm{\pi }\stackrel{\scriptscriptstyle\mathrm{def}}{=}\frac{c}{d}$
where
    c is the circumference of a circle
    d is the diameter of a circle

π is an irrational number and can be expressed approximately as:

<math><mi mathvariant="normal">&#x03C0;</mi>
  <mo>&#x225D;</mo><mn>3.14159&#x2026;</mn></math>
    $\mathrm{\pi }\stackrel{\scriptscriptstyle\mathrm{def}}{=}\mathrm{3.14159\dots }$

e - e (U-0065 or e), also called Euler's number, is defined as the limit of compounding growth:

<math><mi mathvariant="normal">e</mi><mo>&#x225D;</mo>
  <munder><mi>lim</mi><mrow><mi>n</mi><mo>&#x2192;</mo><mn>&#x221E;</mn></mrow></munder>
  <msup><mrow><mo>(</mo><mn>1</mn><mo>+</mo>
    <mfrac><mn>1</mn><mi>n</mi></mfrac><mo>)</mo></mrow>
    <mi>n</mi></msup></math>
    $\mathrm{e}\stackrel{\scriptscriptstyle\mathrm{def}}{=}\underset{n\to \mathrm{\infty }}{\lim }{(1+\frac{1}{n})}^n$

e is an irrational number and can be expressed approximately as:

<math><mi mathvariant="normal">e</mi>
  <mo>&#x225D;</mo><mn>2.71828&#x2026;</mn></math>
    $\mathrm{e}\stackrel{\scriptscriptstyle\mathrm{def}}{=}\mathrm{2.71828\dots }$

i - i (U-0069 or i), also called the imaginary unit, is defined as the square root of -1:

<math><mi mathvariant="normal">i</mi><mo>&#x225D;</mo>
  <msqrt><mn>-1</mn></msqrt></math>
    $\mathrm{i}\stackrel{\scriptscriptstyle\mathrm{def}}{=}\sqrt{-1}$

φ - φ (phi, U-03C6, φ), also called the golden number, is defined as:

<math><mi mathvariant="normal">&#x03C6;</mi><mo>≝</mo>
  <mfrac><mrow><mn>1</mn><mo>+</mo><msqrt><mn>5</mn></msqrt></mrow>
  <mn>2</mn></mfrac></math>
    $\mathrm{\phi }\stackrel{\scriptscriptstyle\mathrm{def}}{=}\frac{1+\sqrt{5}}{2}$

The golden number is actually the positive root of the following equation:

<math><msup><mi mathvariant="normal">&#x03C6;</mi><mn>2</mn></msup>
  <mo>&#x2212;</mo><mi mathvariant="normal">&#x03C6;</mi>
  <mo>&#x2212;</mo><mn>1</mn><mo>=</mo><mn>0</mn></math>
    ${\mathrm{\phi }}^2-\mathrm{\phi }-1=0$

φ is an irrational number and can be expressed approximately as:

<math><mi mathvariant="normal">&#x03C6;</mi>
  <mo>&#x225D;</mo><mn>1.618034&#x2026;</mn></math>
    $\mathrm{\phi }\stackrel{\scriptscriptstyle\mathrm{def}}{=}\mathrm{1.618034\dots }$

⇒ List of Well-Known Functions

⇐ Identifiers of Well-Known Entities

⇑ ISO 80000-2 Quantities and Units - Mathematics

⇑⇑ MathML (Mathematical Markup Language)