What are well-known operators?

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

Square Root - Square root is identified by the Square Root symbol √ (U+221A, &#x221A; &sqrt;), which is represented by the <msqrt> tag. For example:

<math><msqrt><mo>&#x2212;</mo><mn>1</mn></msqrt>
  <mo>=</mo><mi mathvariant="normal">i</mi></math>
    $\sqrt{-1}=\mathrm{i}$

Running Sum - Running sum is identified by the N-Ary Summation symbol ∑ (U+2211, &#x2211; or &sum;) with a running index, index range and index based expression. For example:

<math><munderover><mi>&#x2211;</mi><mrow><mi>i</mi><mo>=</mo><mn>1</mn></mrow>
  <mi>n</mi></munderover><mi>i</mi><mo>=</mo><mfrac><mrow><mi>n</mi>
  <mo>&#x2062;</mo><mo>(</mo><mi>n</mi><mo>+</mo><mn>1</mn><mo>)</mo></mrow>
  <mn>2</mn></mfrac></math>
    $\sum _{i=1}^{n}i=\frac{n(n+1)}{2}$

Limit of Function - Limit of a function is identified as "lim" with a variable limit and function of the variable. For example:

<math><munder><mi>lim</mi>
  <mrow><mi>x</mi><mo>&#x2192;</mo><mn>1</mn></mrow></munder>
  <mfrac><mrow><mi>sin</mi><mi>x</mi></mrow><mi>x</mi></mfrac>
  <mo>=</mo><mn>1</mn></math>
    $\underset{x\to 1}{\lim }\frac{\sin x}{x}=1$

Derivative of Single-Variable Function - Derivative of a single-variable function is identified in 3 options:

Leibniz's Notation - Derivative can be identified in a fraction form with letter d on the function and variable identifiers. Letter d should be treated as an operator using the <mo rspace="0"> tag without trailing space. For example:

<math><mfrac><mrow><mo rspace="0">d</mo><mi>f</mi></mrow>
  <mrow><mo rspace="0">d</mo><mi>x</mi></mrow></mfrac>
  <mo>=</mo><mn>2</mn><mi>x</mi>
  <mtext>, where</mtext><mspace width="0.3em"/><mi>f</mi><mo>=</mo>
  <msup><mi>x</mi><mn>2</mn></msup></math>
    $\frac{df}{dx}=2x\text{, where}f=x^2$

Lagrange's Notation - Derivative can be identified by a superscript of the prime symbol ′ (U+2032; &#x2032; or &prime;) on the function identifier. For example:

<math><msup><mi>f</mi><mo>&#x2032;</mo></msup>
  <mo>=</mo><mn>2</mn><mi>x</mi>
  <mtext>, where</mtext><mspace width="0.3em"/><mi>f</mi><mo>=</mo>
  <msup><mi>x</mi><mn>2</mn></msup></math>
    $f^{\prime }=2x\text{, where}f=x^2$

Finite Increment - Finite increment of a function is identified by the Increment symbol ∆ (delta, U+2206 or &#x2206;), which is not the Greek capital letter delta Δ (U+0394, &#x0394; or &Delta;). For example:

<math><mfrac><mrow><mi>&#x2206;</mi><mi>f</mi><mo>(</mo><mi>x</mi><mo>)</mo>
  </mrow><mrow><mi>&#x2206;</mi><mi>x</mi></mrow></mfrac>
  <mo>=</mo><mfrac><mrow><mi>f</mi><mo>(</mo><mi>x</mi>
  <mo>+</mo><mi>&#x2206;</mi><mi>x</mi><mo>)</mo>
  <mo>+</mo><mi>f</mi><mo>(</mo><mi>x</mi><mo>)</mo>
  </mrow><mrow><mi>&#x2206;</mi><mi>x</mi></mrow></mfrac></math>
    $\frac{∆f\left(x\right)}{∆x}=\frac{f(x+∆x)+f\left(x\right)}{∆x}$

Infinitesimal Variation - Infinitesimal variation of a function is identified by the lower case letter δ (U+03B4, &#x03B4; or &delta;). For example:

<math><mfrac><mrow><mi>&#x03B4;</mi><mi>f</mi><mo>(</mo><mi>x</mi><mo>)</mo>
  </mrow><mrow><mi>&#x03B4;</mi><mi>x</mi></mrow></mfrac>
  <mo>=</mo><mfrac><mrow><mi>f</mi><mo>(</mo><mi>x</mi>
  <mo>+</mo><mi>&#x03B4;</mi><mi>x</mi><mo>)</mo>
  <mo>+</mo><mi>f</mi><mo>(</mo><mi>x</mi><mo>)</mo>
  </mrow><mrow><mi>&#x03B4;</mi><mi>x</mi></mrow></mfrac></math>
    $\frac{\delta f\left(x\right)}{\delta x}=\frac{f(x+\delta x)+f\left(x\right)}{\delta x}$

Partial Derivative - Partial derivative of a multi-variable function is identified by the Partial Differential symbol ∂ (U+2202, &#x2202; or &part;). For example:

<math><mfrac><mo>&#x2202;</mo>
  <mrow><mo>&#x2202;</mo><mi>x</mi></mrow></mfrac>
  <mrow><mo>(</mo><msup><mi>x</mi><mn>2</mn></msup><mo>&#x2062;</mo>
  <msup><mi>y</mi><mn>2</mn></msup><mo>)</mo></mrow>
  <mo>=</mo><mn>2</mn><mi>x</mi><mo>&#x2062;</mo>
  <msup><mi>y</mi><mn>2</mn></msup></math>
    $\frac{\partial }{\partial x}(x^2{y}^2)=2x{y}^2$

Integral (Antiderivative) - Integral of a function is identified by the Integral symbol ∫ (U+222B, ∫ or &int;). For example:

<math><mo>&#x222B;</mo><mi>sin</mi><mo>(</mo><mi>x</mi><mo>)</mo>
  <mo rspace="0">d</mo><mi>x</mi><mo>=</mo>
  <mo rspace="0">&#x2212;</mo><mi>cos</mi><mo>(</mo><mi>x</mi><mo>)</mo>
  <mo>+</mo><mi>C</mi></math>
    $\int \sin \left(x\right)dx=-\cos \left(x\right)+C$

Finite Integral - Integral of a function is identified by the Integral symbol ∫ (U+222B, ∫ or &int;). For example:

<math><msubsup><mo>∫</mo><mn>0</mn><mi>&#x03C0;</mi></msubsup>
  <mi>sin</mi><mo>(</mo><mi>x</mi><mo>)</mo>
  <mo rspace="0">d</mo><mi>x</mi><mo>=</mo><mn>2</mn></math>
    ${\int }_0^{\pi }\sin \left(x\right)dx=2$

Definite Integral Evaluation with Antiderivative - If you know the antiderivative of a function, you can express its definite integral with the vertical bar notation. For example:

<math><msubsup><mo>∫</mo><mi>a</mi><mi>b</mi></msubsup>
  <mi>f</mi><mo>(</mo><mi>x</mi><mo>)</mo>
  <mo rspace="0">d</mo><mi>x</mi><mo>=</mo>
  <mi>F</mi><mo>(</mo><mi>x</mi><mo>)</mo>
  <msubsup><mo lspace="0">|</mo><mi>a</mi><mi>b</mi></msubsup>
  <mtext>, where</mtext><mspace width="0.3em"/>
  <msup><mi>F</mi><mi>&#x2032;</mi></msup><mo>=</mo><mi>f</mi>
  </math>
    ${\int }_a^{b}f\left(x\right)dx=F\left(x\right){|}_a^b\text{, where}{F}^{\prime }=f$

⇒ Function Arguments - "Invisible Apply Function" Operator U+2061

⇐ List of Well-Known Functions

⇑ ISO 80000-2 Quantities and Units - Mathematics

⇑⇑ MathML (Mathematical Markup Language)