Dennis–Stein symbol

Summary

The Dennis–Stein symbol $\langle a, b \rangle$ is an element of $K_2(R)$ attached to two elements $a, b$ of a commutative ring $R$ such that $1 + ab$ is a unit. Unlike the Steinberg symbol, it does not require $a$ or $b$ to be units. It is the convenient kind of symbol for rings with many non-units, such as local rings and rings with nilpotent elements.