Embeddability of real and positive operators

Embedding discrete Markov chains into continuous ones is a famous open problem in probability theory with many applications. Inspired by recent progress, we study the closely related questions of embeddability of real and positive operators into real or positive $C_0$-semigroups, respectively, on finite and infinite-dimensional separable sequence spaces. For the real case we give both sufficient and necessary conditions for embeddability. For positive operators we present necessary conditions for positive embeddability including a full description for the $2\times 2$-case. Moreover, we show that real embeddability is topologically typical for real contractions on ${\mathrm{\ell }}^2$.