For my masters thesis, it’s required to typeset vectors with a single and matrices with double underlines. Whats the best way to accomplish this double underline, as math.underline has no stroke option.
You should be able to use package mannot with its function markul to do this. Either once with a custom double stroke pattern (more trouble than it’s worth, I think) or using the function twice with different values for bottom outset, which is how you move the line.
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A simple solution that gets kind of close to your image involves just underlining twice:
#let ul(body) = $underline(body)$
#let uul(body) = $underline(underline(body))$
$ ul(y)(t) = uul(C)_C ul(x)(t) + uul(D)_C ul(u)(t) $
Defining the ul() function is not necessary, I just wanted to keep the names similar.
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Based on @bluss’s recommendation to use mannot:
#import "@preview/mannot:0.3.0": markul
#let ul(body) = $underline(body)$
#let uul(body) = $markul(markul(body, outset: #(bottom: 0.3em, rest: 0em)))$
$ ul(y)(t) = uul(C)_C ul(x)(t) + uul(D)_C ul(u)(t) $
Double stroke spacing can be controlled by changing the 0.3em to something else.
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thanks a lot! using the approach with mammot’s markul now as the spacing is a bit better than the just calling the standard underline twice.
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You could also use an accent. Note however that under accents in math only work properly in the dev version, not 0.13.1 (IIRC). There’s U+0333 Combining Double Low Line in Unicode, which you can use as follows
#let ull(base, size: 100%) = math.accent(base, "\u{0333}", size: size)
$ ull(C)_C $
(I have no idea if this is stretchy in math fonts, so I’ve just added the size parameter for completeness).