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Norton's Theorem

Any linear two-terminal circuit can be replaced by an equivalent circuit consisting of a single current source (INI_{N}) in parallel with a single impedance (ZNZ_{N}). Vth=INZthV_{th} = I_{N} Z_{th}.
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The statement of the theorem

Let NN be a linear, passive, two-terminal network characterized by its admittance matrix Y\mathbf{Y}. Define the terminal voltage Vab(t)V_{ab}(t) and the resulting current Iab(t)I_{ab}(t) such that Iab(t)=Vab(t)Zab(t)I_{ab}(t) = \frac{V_{ab}(t)}{Z_{ab}(t)}, where Zab(t)Z_{ab}(t) is the generalized impedance. The theorem asserts that NN is equivalent to a parallel combination of a current source INI_N and a resistor RNR_N if and only if the following relationships hold:\n\n1. The equivalent resistance RNR_N is defined by the open-circuit impedance: RN=Zab(t)∣Vab(t)=0R_N = Z_{ab}(t) \big|_{V_{ab}(t)=0} \n\n2. The Norton current INI_N is defined by the short-circuit current: IN=Vab(t)Zab(t)∣Vab(t)=0I_N = \frac{V_{ab}(t)}{Z_{ab}(t)} \bigg|_{V_{ab}(t)=0} \n\n3. For any arbitrary time-dependent voltage source Vab(t)V_{ab}(t), the current Iab(t)I_{ab}(t) flowing through the network NN satisfies the superposition principle derived from the equivalent Norton circuit: Iab(t)=IN+Vab(t)RNI_{ab}(t) = I_N + \frac{V_{ab}(t)}{R_N}
Source: Wikipedia