6.20 Fractions (分数)

Synopsis

The 分数 (fraction) forward reference typesets a short run as a fraction — a numerator over a denominator — rather than as inline text. It is the mathematical-typography sibling of the script glyphs (§6.16): a forward reference with the は particle and a single target, yielding an emphasis node. The official 注記一覧 lists it among the emphasis-page treatments ([AOZORA-ANNOTATION], etc.html); the corpus attests only the forward-reference form.

Notation

Forward reference only — quote the target; the treatment applies to the most recent preceding occurrence (§7.5). The particle is は. The target is a fraction expression separated by a slash, either ASCII / (U+002F) or fullwidth ／ (U+FF0F); both widths occur in the corpus.

fraction = target "［＃「" target "」は分数］"
target   = numerator slash denominator
slash    = "/" / "／"

1/4［＃「1/4」は分数］
１／４［＃「１／４」は分数］

Unlike 太字/斜体 (§6.12) there is no bare inline range and no ここから/ここで block range for 分数: the official guide gives only the forward-reference form, and the corpus attests only it. A comma-joined compound that pairs 分数 with another treatment in one bracket (［＃「3」は上付き小文字、「1/143」は分数］) is not this single-target shape; it degrades to an unrecognised directive (§6.14) pending the multi-directive grammar.

Parameters

target — the single quoted run to typeset. As with §6.16, the form names exactly one target; a multi-quote body is not a recognised shape and degrades (§6.14). The renderer splits the target on the first slash: text before the slash is the numerator, text after it the denominator.

Semantics

The form resolves its target by the look-back rule of §7.5 and yields an emphasis node over that run. It is an inline construct.
The node carries the 分数 attribute so serialization (§7.6) can reconstruct the source directive, including the redundant preceding target copy, byte-exact.
Reference rendering (§8): the target is split on the first slash into a numerator and a denominator, joined by the fraction slash ⁄ (U+2044) —
```
分数 → NUM⁄DEN
```
The aozora-bunsu wrapper distinguishes a fraction from the lone  of §6.16 (aozora-uwatsuki); neither collides with the / of §6.12, the  of bouten (§6.2), or the <ruby> of §6.1. A target with no slash (not attested) renders verbatim inside the wrapper.

Error conditions

Target with no referent — a は分数 directive whose quoted target does not occur in the preceding text has no earlier run to treat, so the quoted target is itself the treated run: a self-contained forward reference, rendered as the fraction wrapper above. Serialization reconstructs the directive byte-exact (§7.6); no input is lost (parallel to the §6.16 rule).

Conformance vectors

fraction_forward, fraction_forward_no_referent (under conformance/vectors/).

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