\mathfrak{y} is used instead of \hat{y} in html / epub, because the \hat command caused jagged rendering with characters above and below the baseline.
\mathfrak{y} doesn't look very like a y at all though. It's needed as the symbol for "predicted output" (see e.g. https://www.statisticshowto.com/y-hat-definition/).
Alternatives could include y\prime but L(y, y\prime) would look more confusing to physicists because Lagrangians.
\mathfrak{y} is used instead of \hat{y} in html / epub, because the \hat command caused jagged rendering with characters above and below the baseline.
\mathfrak{y} doesn't look very like a y at all though. It's needed as the symbol for "predicted output" (see e.g. https://www.statisticshowto.com/y-hat-definition/).
Alternatives could include y\prime but L(y, y\prime) would look more confusing to physicists because Lagrangians.