No CrossRef data available.
Published online by Cambridge University Press: 30 October 2024
Several authors have studied homomorphisms from first homology groups of modular curves to $K_2(X)$, with
$X$ either a cyclotomic ring or a modular curve. These maps send Manin symbols in the homology groups to Steinberg symbols of cyclotomic or Siegel units. We give a new construction of these maps and a direct proof of their Hecke equivariance, analogous to the construction of Siegel units using the universal elliptic curve. Our main tool is a
$1$-cocycle from
$\mathrm {GL}_2(\mathbb {Z})$ to the second
$K$-group of the function field of a suitable group scheme over
$X$, from which the maps of interest arise by specialization.