Make equations numbered by chapters in Typst.

typst-equation-numbering-rules.typ

/// Make equations numbered by chapters (heading level 1)
///
/// Only works if headings are numbered.
///
/// # Usage
///
/// ```
/// #show: equation-numbering-rules
/// #set heading(numbering: "1.")
///
/// $ sin x $ <eq:sin>
///
/// @eq:sin
/// ```
///
/// # Credit
///
/// Modified from Anton Wetzel's comment:
/// https://github.com/typst/typst/issues/1768#issuecomment-1921081584
///
/// https://gist.github.com/YDX-2147483647/4196137b5eaa7779d8ddd114ec5f70fb
#let equation-numbering-rules(body) = {
  let equation-label(heading, equation) = [(#heading#sym.dash.en#equation)]
   
  // Define the `numbering` of `equation`s
  set math.equation(
    numbering: equation => locate(
      // Locate where we are in the document
      loc => {
        // Get the heading index at the location at offset 0 for heading level 1
        let heading-index = counter(heading).at(loc).at(0)
        equation-label(heading-index, equation)
      },
    ),
  )
   
  // Overwrite how a `ref` is displayed
  show ref: it => {
    // Skip if the label has no corresponding element
    // or it is not an `equation`.
    if it.element == none or it.element.func() != math.equation {
      return it
    }
     
    locate(
      loc => {
        // location of the `equation` that the `ref` points to
        let equation-location = query(it.target, loc).first().location()
         
        let heading-index = counter(heading).at(equation-location).at(0)
        let equation-index = counter(math.equation).at(equation-location).at(0)
         
        it.element.supplement + [ ] + equation-label(heading-index, equation-index)
      },
    )
  }
   
  // Reset `equation`'s counter if a new chapter starts
  show heading.where(level: 1): it => {
    counter(math.equation).update(0)
    // display the heading without changes
    it
  }
   
  body
}

Example

#import "where-you-save-it.typ": equation-numbering-rules

#set page(width: auto, height: auto)

#show: equation-numbering-rules

// Only works if headings are numbered.
#set heading(numbering: "1.")

$ sin x $ <eq:sin>

@eq:sin is the sine function.

= Chapter

$ cos x $

Alternatives

• (Recommend) https://discord.com/channels/1054443721975922748/1054443722592497796/1088839394665955378 by laurmaedje

  #let equation-numbering-rules(body) = {
    // Or `show heading.where(level: 1): …`
    show heading: it => it + counter(math.equation).update(())
    set math.equation(numbering: (..nums) => {
      // Or `counter(heading).get().at(0)`
      let h = counter(heading).display("1") // Remove the trailing dot
      let eq = nums.pos().map(str).join(".")
      
      [(#h#sym.dash.en#eq)]
    })
     
    body
  }

  

  #show: equation-numbering-rules
  
  // Only works if headings are numbered.
  #set heading(numbering: "1.")
  
  #set page(width: auto, height: auto)
  
  $ sin x $ <eq:sin>
  
  @eq:sin is the sine function.
  
  = Chapter
  
  $ cos x $
  
  @eq:sin is still the sine function.

• https://typst.app/universe/package/i-figured

  Configurable figure and equation numbering per section.

  Caveat: ref is not supported.

Relevant issues

• Ergonomic numbering-by-section · Issue #1896 · typst/typst
• Dependent numberings · Issue #2652 · typst/typst