We study the observability of linear differential systems in sufficiently arbitrary differential rings; observability is treated in the classical sense, i.e., as the injectivity of the "initial condition-output vector" mapping. A number of observability conditions is obtained. We define observability by finitely many measurements, where a measurement is the value of a homomorphism into the ring of constants on the output variable. We show that observability is equivalent to observability by finitely many measurements. [PUBLICATION ABSTRACT]
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