A Horrocks-type theorem for even orthogonal $K_2$.

We prove a $\mathbb{P}^1$-glueing theorem for even-dimensional orthogonal Steinberg groups. This result transfers to the orthogonal case an earlier result of M. Tulenbaev and is also an analogue for the functor $K_2$ of a 1982 result of A. Suslin and V. Kopeiko.

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